If you have a digital thermostat and the display is blank, one thing to try first is replacing its batteries. Most thermostats are powered by the air handler, getting voltage through the connecting thermostat wire, but some are not and rely solely on battery power. So if your display is blank, try replacing your batteries first. If the display returns but nothing happens after a few minutes you most likely have a voltage issue or bad thermostat. Keep in mind that most digital thermostats have a 5 minute time delay. Allow 5 minutes to see if your system starts working properly. If replacing the batteries didn't help, you will need a service technician to troubleshoot your system.