The answer to 'my AC won't turn on' is that there are several common causes, including a tripped circuit breaker, a clogged air filter, or a problem with the thermostat. First, check the circuit breaker and reset it if it has tripped. Then, inspect the air filter and replace it if it's dirty. If those don't solve the issue, the thermostat may need to be adjusted or replaced.
If the AC still doesn't turn on after trying those basic troubleshooting steps, there could be a more serious problem like a refrigerant leak, compressor failure, or electrical issue. In those cases, it's best to call a professional HVAC technician to diagnose and repair the problem.