I just changed my oil and Now the car won't start.

Check your battery connections at the starter. It may not be getting the needed volts. Just make sure they're clean and tight at the starter and the battery both. Good luck!

Insert the key into the ignition and turn it to the "ON" position. There is no need to start the car.

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Press the button located in the center of the instrument cluster ONCE. The display should now read "OIL LIFE" instead of vehicle mileage.

Read more: How to Reset the oil life monitor on a Saturn Ion ' eHow.com http://www.ehow.com/how_5658600_reset-life-monitor-saturn-ion.html#ixzz28jXOUbyz

do you here the fuel pump working when you turn the key?If not could be a fuse that runs the fuel pump.

Done cars have a kill switch if they are low on oil. Check your oil. If the oil is good, it may be the ignition switch.

If you have steering wheel controls you can reset it by getting to the oil life indicator (mine is on the left side lower button menu when wheel is facing forward). Then press the arrow button for a few seconds to reset it. If you don't have steering wheel controls you can reset it by turning key to on position but don't start vehicle. Press gas pedal all the way to the floor three times (some say 6 times). Turn key off. Oil life should be reset went you start engine next time. I hope this works for you.

The 2.7 L is a chain driven DOHC engine. No belt. Chain should last the life of the engine.
The 2.7 also has a "bad reputation" for going bad - but this is in almost all cases due to the oil change intervals. Change oil every 5,000 miles and sludge won't build up. Don't change the oil and about 15,000 to 25,000 miles later, <boom> bye bye engine.
Not sure where the water pump is. Never had to replace one yet.

Are you absolutely sure that you put the right weight oil back in the car? These cars are very sensative to this and if it doesn't see the correct oil pressure it won't start beacause it uses oil pressure to fire the fuel injectors. You need to be using 505.01 oil in your car only. Anything else can cause a no start or major engine damage!

It depends on your mileage. It's normal for the pressure to read high on start up and then drop some but your numbers are a little low on start up, usually on the high side of 40-45.You could check your pressure with another guage and see if it reads any different. If it does then you should change yours. But if your mileage is running on the high side of 200,000 then it's probably wear and tear. I would be inclined to try a heavier oil, 15-40 or 20-50. This should bring the pressure up and help with lubrication. You could also add something like Lucas to your oil to help with lubrication and pressure. It's also possible the oil pump is wearing out but unless the mileage is below 150,000 it likely won't do much good and a new pump won't like the heavier oil.
If you are unsure of the mileage you could do a compression check. If it's 100 or under the engine is worn.There is still life in it as long as the numbers are close together but it is time to start thinking about the next step.

my truck won't start when it's cold outside. but when it's warm it start's right up I changed the oil and filter and the battery is new what's wrong.

change the starter motor.