Some of the basics you can re-check are:
1) Check the UTH over for any damage where the cord is connected.
2) Make sure it is plugged into the thermostat all the way.
3) You can also plug the UTH into the wall to make sure it is working.
4) If the thermostat is plugged into a power strip, make sure the switch is still turned on.
5) Verify that the wall outlet is still "live". Sometimes the circuit breakers can trip. Or you mave have a Master outlet that popped....