keeps stopping for like an hour then starts again. How dangerous is this? ty

There's lots fans in pc, depends on which one is stopping.
Cpu & gpu fans may get u overheated and u should replace it asap.If case fans not working properly, it won't be that urgent.
Still u need to replace it soon or later. It'll be much easier.

replace it, if he doesnt know it's likely that it was put there by a system builder where it is one of hopefully two fans.