One time, my RPM's stopped going more than 2000RPM and took a week to figure out the fuel shutoff lever/cable was was badly frayed and my engine stop lever was never fully opening the fuel.
Another time, I was working on the cockpit instrument panel, and afterwards, tried to start the motor, and it wouldn't. I spent a couple hours trying to figure out why, only to find the T handle was still up (covered up by the pulled out re-worked instrument panel).
Not saying any of that is related, just that most of the time it is a simple problem and solution.