I had an odd fault on my desktop recently and couldn't get it to do anything. I was advised to disconnect the power supply from the tower. (Not just switch it off) Press the start button for 10 seconds, then connect the power supply and start up. It worked. I was certain that I'd had a hard disc failure, but it wasn't.
For a laptop similar to the above but remove the battery completely and press the start button for 10 seconds.
In over 20 years of computing I've never heard of the above but it worked.