The answer to 'air conditioner not turning on' is that there are several common causes, including a tripped circuit breaker, a faulty thermostat, or a problem with the condenser unit. The air conditioner may not be turning on due to a lack of power, a problem with the control system, or a mechanical issue with the compressor or fan.
If your air conditioner is not turning on, the first thing to check is the circuit breaker. Make sure the breaker hasn't tripped and that power is being supplied to the unit. You can also check the thermostat to ensure it is set to 'cool' and the temperature is set lower than the current room temperature. If the breaker and thermostat are working properly, the issue may be with the condenser unit itself, which will likely require a professional HVAC technician to diagnose and repair.