DarthJake

In my experience it can sometimes be a loose wire inside of the case, not just the PSU cable as iNoa97 suggested (yet that is still a very logical thing to check).  I would suggest checking the data connectors for your drives and perhaps all the other cables to be safe.  Also, this wouldn't happen to be a laptop by chance, would it?

check the PSU cable, they're usually loosely plugged and don't give much to the socket. Try firmly putting the cable in and make sure it has slack. If thats the problem just buy a longer one, they're pretty cheap.