Our digital thermostat is blank.

You may have lost power to the furnace or blown a fuse in the furnace, also it you have changed the filter lately you may not have put the panel back on the furnace properly to depress the door switch. Good luck, Mike

Actec answers are good ones. I would also replace the batterys if your thermostat has them.

Post the model number of your unit, if you still have problems...