I assume you have a split system, outdoor condensing unit with indoor furnace...there will be a 24 volt control transformer, typically located in the condensing unit wiring compartment, but could be located within the furnace.
The 24 volt control transformer will have protection in the form of fuses or sometimes an integral circuit breaker with built-in reset switch. The first thing I would check is that there is 24 volt control power present. You can check this right at the secondary of the transformer or between the 'hot' and 'common' wires, typically the red and blue wires at the thermostat. Sounds like you have lost 24 volt control power from the transformer, if so, reset the breaker built into the transformer or replace the fuse just ahead of the transformer, whichever you have.