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Do you know model on thermostat and has anything happened such as work being performed or electrical power been off?
OK usually there are two reasons for losing display. If battery operated low batteries, otherwise loss of power from furnace.
Can you do some hands on troubleshooting and do you have a multimeter for checking voltage?
The furnace sends 24v to the thermostat.Your furnace has a small led light inside on the control board. Can you look in furnace and see if the red light is on solid?If you have to remove panel to see light it will be necessary to hold door switch in to have power to board.
You will not need a multi meter to look to see if the light is on control board.
Look and see if there is a small auto type fuse on control board. Remove the fuse and see if it is blown
You need to remove the panel to get to the board
Fuse looks good. Next lets follow the control wires coming from the furnace board going to the thermostat.See if perhaps the red wire is routed through a drain float switch that may be breaking the circuit.If so lift the float and see if we get display on thermostat.