I assume you have a digital thermostat?
If so, and it's an OEM thermostat like this:
it's blank because it isn't getting 12 VDC from the furnace. There is an On/Off/Reset switch on the furnace behind the outer panel It should have 12 VDC both to the in and the out of the switch IF you have 12 VDC on the house DC buss and the appliance fuse on your converter/breaker panel is not blown.
If it is an aftermarket digital thermostat like this, it may just need the AA batteries replaced.