Computer random restart

Hey guys, i've been having problems with my computer lately
since there was a power-shutdown in my city earlier this week, my pc has started working strangely... often when i log off my pc, and let it go to sleep (after 30min of inactivity) it "shuts down" to sleep mode, and when i press any key it starts, you know..... but now when i've logged in or woke it up from sleep, i restarts in a couple of secs.. and it's really annoying, and the other day when i was playing LoL in the middle of loading my computer just restarted :/
could the power-shutdown have done something to my computer? (it was in sleep mode when the power was gone..) could there be something with the psu? or is this still fixable? (gonna update my build in the beginning of next year, but don't have the money now so don't wanna start upgrading now..)

try checking the computers 'Event Viewer' from the control panel => Administrative Tools & to see if you find any errors which either match the time your computer is restarting and maybe pinpoint why this is happening. Could be that it's an application causing it or some other service that is not loading or shutting down properly.