To troubleshoot a LiftMaster garage door opener that won't open, first check if the opener is receiving power by verifying that it's plugged in and the circuit breaker hasn't tripped. Next, examine the safety sensors to ensure they are aligned and free of obstructions. Check the remote batteries and try the wall switch to rule out remote issues. Inspect the door itself for physical obstructions or damage. Review the opener's diagnostic codes for specific issues. If the problem persists, consult the manual or a professional for further assistance.