DIAGNOSIS The internal protection timer has locked out the system. This happens after many short cycles, usually from a dirty air filter or a blocked condensate drain.
FIX Replace your furnace air filter. Clear the HVAC condensate drain line. Then, to reset the lockout, press and hold the Fan button for 5 seconds. The screen will refresh and normal operation will resume.
IF THAT DIDN'T WORK The lockout was caused by a flame sensor or ignition failure. You need to check the furnace error code on its control board. The thermostat is just reporting the system's fault.
HEADS UP Ignoring the root cause will trigger the lockout again. If your filter is clean, a technician needs to diagnose the furnace's safety circuit.