My heating/cooling unit works fine, but the lcd window is not working. Is there a battery that needs to be replaced within the thermostat?

No display normally means you have no 24 volts to the thermostat, you have no common wire to the thermostat or you have a faulty thermostat. If you have a volt meter check across the "C" and "R" terminals at the thermostat and see if you have 24 volts, if it is a battery operated thermostat, the batteries are for back up if you should loose power to the thermostat. Good luck, Mike