The answer to 'AC not turning on' is that there are several common causes, including a tripped circuit breaker, a faulty thermostat, a problem with the condenser unit, or a lack of power to the system. Symptoms may include no air flow, no cooling, or the AC unit not responding at all when the thermostat is adjusted.
If the AC is not turning on, the first step is to check the circuit breaker and reset it if it has tripped. You should also check the thermostat to ensure it is set to 'cool' and the temperature is set lower than the current room temperature. If those basic checks don't resolve the issue, it's likely a more complex problem that will require a professional HVAC technician to diagnose and repair.