Sounds like a bad thermostat. First test it by making a "jumper" cable. 2 alligator clips connected by a wire.(wear rubber gloves to prevent shock) Remove the cover to expose the wiring. Touch one end to the R- RED terminal and the other to the Y-YELLOW terminal. Your A/C should kick on. If your A/C works the thermostat is the problem. Buy another at the hardware store. BEFORE you replace it cut power from the breaker box. (Keep the gloves on just in case.) Their are 4 terminals on the thermostat with color coded wires attached. R-Red, W-White, Y-Yellow, G-Green. Mount the new thermostat replace wires and restore power. If this solves your problem great; If not call a HVAC tech. Good Luck