Where do I find model number or serial number?I turned off main water valve to replace a leaking pipe and now I can't turn on the water heater. How do I do it?

You have 2 problems with several possible causes:
You have 10-year-old electric water heater that was reinstalled after falling down, and doesn't heat the water.
And you have no hot or cold water pressure, and no pipes are leaking.

1) First problem is lack of water pressure throughout house.
Turn on outdoor spigot nearest to water meter.
This spigot will tell you if problem is inside the house or between the house and meter.
If spigot runs full force, then you probably have clogged pipe where water heater connects to main cold line, or the main shut off valve got clogged with sediment that broke off inside the pipe when you were working on water heater.
Open and close shut off valve to see if that helps. Then disassemble pipes and track back the problem. Put pressure gauge on water heater drain valve to watch pressure drop when turning on indoor faucet.
Read following information:
http://waterheatertimer.org/Low-hot-water-pressure.html

Water heater doesn't work:
Test across wires at top of heater to see if you are getting correct voltage:
If tank is receiving correct voltage, then no hot water puts suspicion on upper element or upper thermostat, or both.
Open following link for electric water heater troubleshoot steps:
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

If valve is dripping, buy garden hose cap and screw onto valve.
Whole valve unscrews counterclockwise.
If valve is plastic, expect it to break off.
If valve is brass, use cheater bar on your pipe wrench.
I don't know that serial number or part number, but drain valves are standard 3/4" pipe thread.
http://waterheatertimer.org/How-to-replace-water-heater-drain-valve.html

Drain valve can be rusted in place and impossible to remove without draining tank and putting water heater on it's side.
Install new water heater so drain valve is accessible, and buy new water heater with brass valve instead of plastic.

If I understand correctly you have residential electric water heater, and your shut off valve located above water heater on cold-water line is leaking?

1) Hot water would come out of this valve if it's leaking because water inside tank is hot and under pressure ... also remember, hot water rises so tank is probably heating the water inside the cold line during hours when no hot water is being used.

2) You probably need to replace the valve. But if water is leaking because a pipe is rusted, the valve may be ok, and just the pipe needs to be replaced.
Take a photo of your problem to hardware store or home center.
I used to work in those stores and could help folks get the right parts.

3) Here are some steps for replacing the valve.
Shut off main water source into house. This is usually located underground in same box where meter-reader looks at meter.
Your water heater shut off valve is called a gate valve, and it will be 3/4"
All water heater pipes are 3/4"
You will need two 10-12" pipe wrenches, maybe a new gate valve, maybe a new flex connector, probably short 3/4 pipe pieces, teflon tape for each threaded pipe.
Hardware store or home center can get you the right materials, just take a photo over to hardware store

4) When you get started, turn off water heater circuit breaker for a couple hours so water will cool down. You don't want to get scalded with hot water.
Do NOT drain the tank.
Simply open a bathtub faucet on hot side to release pressure.
When finished, turn on the water, and make sure water is flowing out of tub spout in full stream before turning circuit breaker ON

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

A) Calculate date of water heater manufacture from serial number located on side of tank:
http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdf
Date of manufacture is not date of installation.
Warranty is usually from date of manufacture
Serial number also shows tank size in gallons, tank volts and watts, and whether tank has 1 or 2 elements.

B) Troubleshoot leaking water heater:
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html
Several things can be looked at to see where leak is coming from.
Carefully go over tank pipes and valves to eliminate every suspect.
Final suspect: tank is rusted out.

C) If tank is rusted out, then tank needs to be replaced before it ruptures.
Ruptured tank can cause major damage and water runs until shut off.
To prevent rupture, turn off power to heater, turn off valve located above tank, open tub faucet for a moment to relieve pressure on tank.
Turn on heater only when needed until new tank is installed.
4500 Watt water heater produces 21 gallons hot water per hour.
Shower uses 6-9 gallons hot water. Bath uses 12-14 gallons hot water.

"It is possible to limp along with a failed water heater if the leak is slow and if you turn off power and water when not using hot water. That trick could buy time, possibly up to 3 months. With time, you can consider how to make the system do more for less as well as comparison shop, get competitive bids, etc"

D) Now you're home is safe from damage.
If tank is only 5 years old, it rusted out pretty quick.

There are three things you can do so next heater lasts longer.

1) Learn about anode rods, and inspect anode rod every 1-3 years.
Loosen anode rod before installing new tank so replacement is easier.
http://waterheatertimer.org/Replace-anode-rod.html

2) Attach electrical ground wire between hot and cold pipes to prevent stray current from reducing life of water heater
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#straycurrent

3) Flush tank each 6 months to prevent sediment
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#flushelectric

The thermostat probe on most residential water heaters is part of the gas valve. The thermostat is a non replaceable part of the gas vale and if your t-stat is bad you will need to replace the gas valve also.
This is generally not a very hard job just start off by shutting gas supply valve to off position.Disconnect main gas line to burner,line to pilot light, and thermocouple from gas valve. Remove pipe fitting from gas valve and use large pipe wrench to loosen gas valve.
Before you loosen gas valve you will need to shut water supply off and drain water from tank.MOST WATER HEATER GAS VALVES ARE PRETTY STANDARD ITEMS AVAILABLE AT MOST HARDWARE STORES. Just make sure you purchase valve for the type of fuel you have,such as l.p. or natural gas.Remove gas valve by turning counterclockwise. Use teflon tape or pipe joint compound on all threaded connections for reassembly. Hook up gas line,burner supply line,pilot and thermocouple lines. Refill tank and check and check for leaks. Light pilot and turn gas control to on. Set to desired heat setting . Also check all gas fitting for leaks. Good luck. Thanks

As long as the heater is still working good and not leaking I would just replace the Temperature and pressure relief valve. To do this turn control knob on gas valve from on to pilot. This will maintain pilot so you won't have to relight later while keeping the burner from firing while you are replaceing t&p valve. Turn off water supply on cold water supply to water heater.If this valve won't work or you don't have one shut off supply to house. Do not attempt to remove t&p valve until you are sure water is off. Open hot water on faucet close to heater and drain a few gallons from water heater drain valve. Purchase new t&p valve at hardware store. They are pretty standard 3/4 inch male pipe thread, 150 p.s.i. rated. Use teflon tape to seal pipe threads ,4-5 wraps at least or pipe joint compound to seal threads You will need a pipe wrench to remove old valve. Turn it counterclockwise to lossen then remove. Have new valve ready to go to minimize water loss. Be carefule as water will still be hot. Install new valve and tighten securly. Turn water back on and check for leaks. Also shut off faucet you opened. Once you are satisfied there are no leaks turn gas valve to on and bleed air from several places. I hope this helps you. Thanks.

It sounds like two problems.
Problem 1) Clog inside faucet or clog inside pipes at faucet
Remove supply tubes leading to faucet and see if problem is inside pipes or inside faucet.
My bet is clog inside faucet.

If you have galvanized pipe that meets copper pipe, this can be trouble spot.
Article on clogged water pipes at Inspectapedia:
http://www.inspectapedia.com/water/Water_Pipe_Clog_Repair_Guide.htm

Article on clogged pipes:
http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html

Problem 2) Water heater leak ... you say tank valve is leaking? Is that cold-water shut-off valve? Or tank drain valve? Or TP valve?

If cold water shut-off is leaking, then replace shut off.

If tank drain valve is leaking, then check if plastic valve is cracked. If drain valve is cracked, then replace with a brass valve from Home Depot. If plastic valve breaks off, then use hammer and screwdriver to gently chip out the broken plastic valve. New valve threads need teflon tape to seal pipe. If valve is just dripping, then put garden-hose cap over end of valve and screw down tight.

If TP valve is leaking, then replace with same temperature and pressure rating. New TP valve threads need teflon tape to seal pipe.

Problem 3) If pressure is low all over house:
Buy pressure gauge at hardware store that screws onto hose connection.

Test pressure on outdoor spigot.
Open faucet and check if pressure drops. This says if problem is inside pipes that enter house.

Test pressure on water heater drain valve.
This says if pressure problem is before -or- after water heater.

If problem is before the water heater, then shut-off valve located on cold water line is suspect.
If problem is after water heater, it could be in hot water outflow pipe on top of water heater, or a nearby elbow.
Remove hot water line leading from water heater and check pressure. This says if problem is where hot water leaves tank.
Read articles above about clogs inside water lines.

What is the leaking pvc pipe connected to? The T & P valve on the side (or top) of the tank? Or is it an incoming cold or outgoing hot water line? If you have just lit the water heater, the T & P valve may drip, but only for a little while and it will stop. The T & P (temperature and pressure relief) valve may be bad and need replaced. Turn off the incoming cold water valve and turn the gas valve to off position. Then drain the water heater using the T & P valve, it will stop draining when the water level is just below it. Remove the T&P valve and replace with new one. Use thread sealing tape on threads. Or call a plumber if not sure.

Replace valve or if it has the usual diaphragm inside remove valve and replace washer/diaphragm
To prevent water draining from the whole system, turn off the taps on the heating pipes located just below the boiler AND the cold mains inlet

try replacing the pressure relief valve to do so turn off water to heater and turn off power or gas which ever you have drain water from heater. then using pipe wrench remove old relief valve. when installing new valve use teflon tape on threads to prevent leaks then thread by hand back into water heater and use pipe wrench to snug dont over tighten turn water back on after full and no leaks turn power back on or relite piolit