We came back from vacation & the thermostat is not displaying anything on the screen. We have check breaker & that is on. We took the face off the upstairs theremostat to see if it would work then & it did not. What could explain this?

Model and brand of thermostat?

Possibly a blown fuse on the printed circuit board in the furnace or air handler.

Totaline P474-0100

See acetec post. It probably is the fuse. NOTE only replace with like same value