DIAGNOSIS: Your thermostat has lost power. This is almost always a wiring or HVAC system issue, not a dead thermostat. The C-wire is the most common culprit.
FIX: First, check your furnace/air handler. Its door safety switch might be disengaged or a fuse/breaker might have tripped. Ensure the furnace door is closed properly and check its 3-5 amp fuse on the control board. If that's fine, the issue is at the thermostat. Remove it from the wall plate and check the C-wire connection. Ensure it's firmly seated in the C terminal on both the wall plate and the furnace control board.
IF THAT DIDN'T WORK: Your HVAC system might have triggered a lockout or hard shutdown. Turn off the power to your entire HVAC system at the breaker for 60 seconds, then turn it back on. This can reset the control board.
HEADS UP: If you have a heat pump, incorrectly wiring the O/B valve can cause the system to trip a safety and cut power. Double-check that configuration in the thermostat's installer settings if the problem recurs.