Transformer

I installed a new thermostat last night without turning the power off, and i have read that doing so may cause the transformer to short out. How do i tell where the transformer is and if that is the problem?

If you have a multimeter, it should be fairly easy. If I remember properly, measure between the red and white wires...if you have 22-30vac, it should be fine.

You would only damage the transformer if you shorted the lines while installing the new thermostat. Just touching two or more wires together would NOT necessarily short out the transformer unless it was the "wrong" two wires. Even then it would normally only happen if the transformer had an internal, or external, fuse. You probably have a miswired thermostat if it is not working.

As to WHERE it is, if you have a forced air unit, air handler, or gas furnace, the transformer is inside that unit.