The answer to 'why is my mini split not turning on' is that there could be several common causes, such as a power supply issue, a problem with the thermostat or control board, or a refrigerant leak. The most common symptoms are the unit not powering on at all or not responding to the thermostat controls.
Some possible solutions include checking the circuit breaker, inspecting the thermostat and control panel for any error codes, and ensuring there are no refrigerant leaks. However, if you're not comfortable diagnosing and repairing the issue yourself, it's best to call a professional HVAC technician who can properly troubleshoot and fix the problem.