Testing which part of the PC has broken.

Thanks for the replies. No he didn't clean it, the last time I asked him to clean the computer he used a full sized vacuum hoover on it (which it remarkably survived), so I just asked him to leave it alone (but I think he turned it on anyway).

I've just tested the power supply using my own. The same thing occurred, the CPU fan span up, but petered out. I forgot to mention that I tested whether it was the fan itself, plugging it into a regular fan port and it did run, albeit at a much slower pace. There were no signs of life from the CPU.

As to SkyNet, I forgot to mention the specs. This is a i5 2500K I built back in 2012, not sure if that constitutes as modern or not.

P.S: full specs:
i5 2500k
GTX 560ti
Z77 Extreme4
Seagate 500GB
Ripjaws 8GB

UPDATE: I managed to get it running! After looking at the motherboards I noticed there were still several spots of dry alcohol on it. After cleaning them off with a cotton bud windows loaded up and I was able to access my Brother's account. The only issue that is still persisting is that the HDD appears to have malfunctioned though it is still running, though at a transfer speed of around 50KB.

I've transferred his files onto a backup HDD and plan on testing the SATA cable and different to see if it's not an issue with that or those. But I might just (have him) buy a new hard drive, which thankfully is a much more cheaper alternative than buying him a whole new computer

Everything seems to be running okay too. The CPU appears to be fine, the RAM appears fine, the GPU is functions though at a decreased resolution, but that might only require reinstalling the drivers, which didn't appear to be installed.

So what steps should I take after this? I didn't utilise any liquids or solvents to clean the MOBO, and I'm wondering if I should give it more through cleaning (I only cleaned the spots I could spot).

Also do you guys have any recommendations for something that could cover the top of the case up? The case was a Corsair 300R and with it's open mesh, I'd be nervous that a similar thing could happen again.

Update 2: After reinstalling Windows the HDD also seems to be working! Thanks for assistance!

I did find these dust filters for the 300R https://www.overclockers.co.uk/demciflex-dust-filter-kit-for-corsair-300r-black-black-cm-049-df.html but does anyone know of a cheaper alternative?