The easiest way is to unplug the furnace for about 15 seconds and then restore power. If the furnace is not accessible, i.e. attic or subfloor, you can gently remove the thermostat from the sub base (try not to pull out the drywall screws). Re-install after 15 seconds. If your furnace is labeled (correctly) in the electrical panel, you can reset the power there for 15 seconds. You will know that it is the right breaker if the thermostat goes blank after you flip the breaker off. Some thermostats have a reset button, check in the installation manual for details. Call us for more information if the above instructions do not resolve the issue.