The thermostat is blank as if it has lost power. Is it broken or is there a wiring problem? I have checked out the power source and it does not appear to be the problem.

Doesnt this run on batteries? are they loose inside or the contacts to them dirty?

This model of Honeywell thermostat uses the 24v from the unit to power it. One of three things must have happened.

1. You have lost power to the furnace, therefore no power to thermostat.
2. You only have 4 wires hooked up on the thermostat and not 5. A lot of older homes had just 4 wire running to the thermostat, which will not work for this thermostat. You should have a wire connected to the (C) or (B) terminal on the thermostat, which ever one they have, I can't remember off hand. These are the Common terminals. They have to have the common from the furnace 24v power to operate.
3. Faulty thermostat, which will obviously have to be replaced.

Hope this helps and gets you to cooler days!

assuming this is a central air unit, you have lost power to the heater giving you a blank display on the thermostat. Also, some thermostats require batteries for the display to work.

Here is a simple way to tell. Remove your thermostat from it's mounting plate, locate the wiring, the red and white are low voltage control wires. Disconnect them from the base and join the two together if the furnace runs you need a thermostat if not it is in the controls to the furnace.

Does the thermostat run off battery power or does the transformer supply power to the thermostat. You can check this by removing the thermostat face and checking for a wire connected to the C terminal. If there is no wire there then it is battery only. Replace the batteries. If you have the brand and model # of the thermostat, you can do a Google search for the owner's manual. Of course there is always the possibility that the thermostat has gone bad.

Check the transformer in the air conditioner/heater. There should be a fuse on teh 24 volt side. Shut power off to the furnace. Locate and check the fuse. If there is no fuse, you probably destroyed the transformer.

Sounds like you have a loose/broken 24volt wire. I would look at the wires comming off the transformer, the board, and the thermostat connections first.

Somewhere you are losing 24 volts. Obviosly the transformer is working, so you are just losing control voltage somewhere.

Check all connections at wirenuts, stackons, etc.

One of two things either the disply has gone out which is likely as it is a common failure for that thermostat or your heating unit is not sending control power. the older chronotherms were system (24volt ac) powered the new hardware/box store units are battery powered. Either way display should not be blank with fresh batteries. to get heat on remove the thermostat jumper or twist together the wire from the R terminal with the wire from the W terminal. note the color of each wire for later reference, heat up house disconnect wires from each other and go purchase a new thermostat

Blue (C) or sometimes (B) should be your common wire. Look at your schematics and locate common. Your Brown wire is (E) and is emergency heat, white wire (W) is Auxilary Heat, green wire (G) is fan relay, red wire (R) is power, yellow wire (Y) is Cooling and orange wire (O) is reversing valve.

since you say it has a blank face it must be a digital thermostat. if the power is on to the furnace and the furnace door is on (door switch) and still does not give you a reading. You probably have a bad stat. if you are handy.... unhook the wires in the thermostat. jump the red and white wires together. the furnace should turn on. if it does the thermostat is bad. if it does not turn on its not the thermostat its the furnace