Monitor doesnt wake up after sleep mode

[pipots]

I have been having trouble with my computer waking up from sleep(2hrs-overnight). When waking it up the power button lights up and doesnt flash(which signals sleep) and then the keyboard lights pop up aswell as the mouse but my monitor stays dead and doesnt power up. Happened like 7x now and ive never had this problem before (computer is almost 3 years old), ive been wondering if ive done something wrong,i have recently replaced my motherboards generic cooler fan which was from a m5a99FX pro r2.0 and installed a coolermaster hyper evo 212 and followed instructions clearly on how to do it on youtube. One big mistake to was i left my house to go to a trip for 3 days and left it on sleep mode aswell (but ive left it 2 days before aswell but no damage or problem or so). Can you guys help me figure it out?

 

[Phillip Corcoran]

I've left my PC in sleep mode for a whole week without any problems - - everything woke up (including monitor) when I returned home a week later.

Shut down your PC then try unplugging both monitor cables (power and video) for 30 seconds, then plug both back in.

Now power up your PC & try sleep/wake up.