Double-check that the thermostat is in the "cool" position and that the temperature is set lower than the ambient temperature in the room. If it doesn't work, make sure the circuit breaker for the air conditioner isn't tripped by checking your circuit breakers. If it has, try again to turn on the air conditioner after resetting the breaker. If none of these fixes work, you might need to contact a specialist to identify and resolve the problem.