I have a digital thermostat bryant and the screen went blank how do I fix it its for a air conditioner

Check the batterys
Check the 24 volt control circuit/transformer.

If you don't have 24 volts its not going to work. Check your breakers going to the blower section(gas heat or furnace)If you don't have line voltage you won't have secondary voltage. The bryant units have a low voltage fuse on the control board that can blow to protect the board(simple automotive fuse). It kills 24 volts to the system. If its blown you can replace it with the 5 amp fuse, if it blows again it time to find out why.