Okay so a few months ago I donated my PC to my cousins (I got a new one) and it worked fine. After around a month they asked how it could work better in games so I recommended an RX 460 or GTX 1050. They went with the 1050, and it worked fine.

Yesterday they called me up and said the computer won't work, and nothing will display on the monitor. I went over there and the computer would turn on, GPU fans spinning, cpu cooler spinning, case fans spinning, lights on, but nothing would show up on the monitor.

First I replaced the DVI cables with spare ones, to see if it would fix it, to no avail. I then tried an older monitor I had, still didn't work. I tried reseating all components (cpu gpu ram etc) and it still didn't work. I cleared the CMOS and also took out the little battery in the mobo and put it back in (I cant remember the name) and still, nothing would come up. I tried taking out the GPU and replacing it with the older one (Nvidia GT 320) and still nothing would happen.

I realised that even the windows sound wouldn't play (the little chime that you hear when you turn your PC on) so I suspected the HDD wasn't working properly. I put in my USB drive and was going to boot from there... but even the BIOS wouldn't show up. As soon as I pressed the power button, the monitor would detect that the PC was on, but show nothing at all and just display "No Signal". I tried taking out the HDD, but it didn't fix the problem.

It was around 6PM and I said that I'd come back tomorrow to try to fix it and I went home. Tomorrow I came back, turned the PC on and it was working. Well, not at first. It would show nothing but after around 15 seconds, Windows was there. They did absolutely nothing to the PC while I was gone (because they're not good with PCs) so I don't understand how it would've magically fixed.

Does anyone know why it wasn't working?

PC specs: -1 TB HDD split into 2 500gb partitions.
-Inno3D GeForce GTX 1050 Compact Graphics Card.
-Intel Core2 Quad Q8300 2.5ghz
-An OEM motherboard from Packard Bell
-6 GB DDR2 800mhz ram

It's now working fine and performance is the same (within margin of error)