Blackscreen after resetting pc

Uberrover

Distinguished
Feb 15, 2017
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So here's my problem...
I was gonna sell my old pc for 350 euros to a good friend of mine, specs:
CPU = AMD A10-7850K 3,7GHz quad core
Mobo = GIGABYTE GA-F288m-HD3
RAM = Corsair DDR3 1600MHz 8gb (1 stick)
GPU = Sapphire R7 260x
PSU = Cooler Master G550M

So, I went ahead and reset my old pc because I was gonna sell it, and I wanted him to get it like new, without any files on the hard drive. So I went ahead and reset the pc.
Right after I clicked "Reset pc" or something among those lines, My screen turned black, so I, as a patient person waited for like half an hour for anything to happen on the screen, but nothing did.
I turned it off using the power button, turned it back on tapping the delete key on my keyboard (on which the lights were on). Nothing.
I started stripping down the pc, removing part by part to see if the pc will work, first HDD, than RAM (btw after the ram it didn't give any beeb on my motherboard, but I'm not sure if it actually should happen or not), and after that the GPU. Still nothing.
When I had the pc on for a while, with a blackscreen, I noticed the LEDs on the front fan (which is the only fan plugged into the motherboard) would sometimes turn off, and then turn back on. The fan seems to be really low on RPM too, as I can easily stop it with 1 finger and it will stay still, until I spin it with my finger. The other fans, powered by the PSU seem fine.

Can anybody please explain me what's wrong with my old pc? I was actually going to give it to him tomorrow, but I think I'm gonna have to postpone that. I personally think it's a motherboard issue, but I'm not sure what to do if so.

Edit: Never overclocked my CPU, did overclock GPU, memory to 1800Mhz and core to 1225MHz if I remember correctly.