Basically, I was working on my pc and then left it for a bit, causing it switch to the standby mode. Then when I tried to work on it again, it wouldn't react on any actions, mouse and keyboard seemed to be disconnected (even though I checked all the connections and they were fine), the screen and power button highlighting would flash like they usually do in standby mode. Switching it off and then back on would lead to the same state. Sometimes CPU would go all loud, like it's trying to get back to work, but then no signs of awakening again.
It's also normally very loud when turning on, if that helps.
I opened the case to see whether there are any blown capacitors and other visible damage but it doesn't seem like so (photo attached).
Any ideas on what the problem might be? I'm not too good with computers so I'm totally confused by this sudden refusal to work.