I once pressed the "sleep" button on my keyboard by mistake.
PC went in sleep mode but it did not "wake up" anymore, no matter what I did.
It was a BIOS issue.
So I simply removed that little battery on the motherboard and then placed it back in.
Afterwards the computer started booted up normally, the only difference being that it made a rollback to an old BIOS version.
So I simply installed the latest update and everything was fixed.

Don't know how good it is to remove that battery thingy but it worked for me.