The tank is empty but the full light stays on

The full light normally comes on when the tank is full. Emptying the tank and re-installing the tank should allow the system to reset. However, if the tank isn't pushed in all the way... the full switch and indicator light will be fooled into reporting a full condition. My suggestion is, first pull out the tank, look at the float and the tank full switch. Become familiar with these two items and confident that 1) the float is not stuck in the up position; 2) the tank full switch that the float is supposed to actuate will toggle (press your finger against it and let it return.) After this exercise make sure the tank is seated all the way in place. If your issue is mechanical, its easier to fix than if it's electrical, and if it is electronic... (just pray that it isn't).