could also be your pc overheating

If it happens on knocks it could be your gpu aswell, like some fragile pins

PC's turning off are most commonly caused by overheating, but could also be a motherboard / gpu issue as mentioned above.

Are you using a laptop? It's probably overheating.

Mine freezes with an image on screen and a loud buzzing of whatever noise was playing just before the crash

I suspect it is GPU related as it only happens in non tf2 games, but idk because its a gtx 770 and it only gets up to like 55 C

this happened to me and it turned out to be my 24pin connector not sitting right for some reason
don't know if that is the issue with your's but i would check all the power cables just to see.

my pc used to do that and i took it to the shop and it turned out i had knocked the psu a little out of its socket when i moved it, worth a try

Hmm okay thanks for the responses guys, much appreciated.

It's a pc I built a few years ago.

What do I do about either issue (overheating and gpu/motherboard)?
I'm a bit mongy with pcs you see

for overheating, try reapplying any thermal compound, especially if it's been there for a couple years untouched
if the problem persists i suggest lowering voltages/clock speeds of your components as well to see if any more thermal throttling happens