I've had an interesting day, rooting around inside my PC.
I noticed earlier this week that my PC was occasionally taking longer than usual to show the motherboard manufacturer logo (the screen with the options to enter the BIOS menu), and my keyboard's backlighting wasn't turning on until this happened (which I guess means the USB ports weren't powered). It was only taking 5 or 6 seconds longer than usual, so I thought little of it. In the last two days it started taking longer and longer, and this morning it must have taken 30 seconds. The PC worked perfectly fine once it posted, and windows 10 loaded up no slower than usual.
Early this afternoon I foolishly tried to fiddle with the overclocking settings in the BIOS. The machine rebooted a few times, and then gave me a screen that said the overclock had failed, and to hit F1 to return to the BIOS. I did this, and tried a lower overclock setting. After this, it took me about two hours to get it to post again. The machine would start up, all of the fans were running (CPU, GPU and case fans), no beep codes, nothing that indicated the machine wasn't running correctly except for the fact that the keyboard lighting wasn't on and nothing was displayed on the monitor. Since then I tried unplugging every USB device that wasn't essential (which was a lot, I think I had 8 things plugged in), using the CMOS reset jumper, removing the CMOS battery (which involved removing the GPU as it was hidden underneath) and waiting.
I eventually got it back into the BIOS, which by now had reverted to factory settings, and the system clock was showing 00:00. I've since reinserted the CMOS battery, and it's still only posting some of the time, and taking longer than usual to do it. Any ideas? I think the overclock attempt is maybe a red herring, as it's been doing this for a week now, and I hadn't touched any BIOS settings in months before that.
Specs: i5-4670, MSI Z97 Gaming 3 motherboard, Corsair CX 750W 80+ PSU, ATI Fury X GPU, 16gb RAM