A/C dead, no 24V power
If you touch it, it should be warm to touch. Check for 24v in AC mode. If it isn't present, see if there is fuse.
After checking the fuse and you still find no voltage, de-energize the unit, isolate 24v transformer and measure resistance across 24v side. If it's open circuit, its burned out.
Transformer is quite reliable, so at this point, you need to make a service call. The transformer didn't likely fail. Something killed it most likely and could be something as simple as chaffed thermostat wire shorting out 24v.