My desktop developed a problem after the last Windows 10 update, where the CPU fan would rev to high then fall to zero every 15 seconds or so. Before that, it had never been a problem. I tried to adjust this with speedfan and didn't have much luck, but a few days later it started working normally again, or so I thought. For the last week though it hasn't come on at all, not even the usual rev-up when you power on. Opened the case and the fan and heatsink all look clean and the fan spins easily with a finger. Any clue how to test what the actual problem is?