Problems With Sleep Mode

This may be in the wrong section, I think the problem may be my motherboard, but I thought this could be a better section since I could be wrong. Anyway a while back I was having a problem that whenever I would try to turn my computer on it wouldn't boot Windows and I'd get just a black screen with a white hyphen at the top left corner. Someone suggested to reset my CMOS, which I did and now it happens, but rarely. Lately though whenever I try to turn my computer back on from sleep it won't wake up.

It's very odd because a lot of the times it never even enters sleep mode, the fans still run and the computer is on but just the display turns off when I put it to sleep. I'll move my mouse or hit a few keys to wake it up and even though I can clearly hear that several components are fully on and not stopped (like I said fans still running cpu cooler etc.) the display won't turn back on so I have to force a shutdown and then often run into the previous problem I mentioned at the top usually after doing so.

I tried resetting the CMOS a few times but it that hasn't fixed the problem. Does anyone have any suggestions for how I can resolve this? And it doesn't always not go into sleep mode, only about half the time, the other half it actually goes to sleep and wakes up perfectly fine so it's pretty odd.

Thank you for any help