My ac was running yesterday and blowing hot air. The fan on the condensing unit locked up. I replaced the fan and the capacitor and the thermostat to that unit went blank. You can get the unit started from outside and I took the faceplate off the thermostat and put interchanged it with the thermosatat on my second unit and it works fine. Why will the thermostat not get any power?

Start looking for broken tstat wires between the furnace?airhandler... if OK check for power to furnace,, is power going to transformer is there low voltage power (24-27 volts coming out of transformer, before you do all this turn off the power at breaker box to furnace, wait a couple minutes turn back on,see if this helps power to tstat,, you didn`t say if the tstat has or needs batteries,, if so change them