Monitor going to sleep before computer starts

Basically my monitor thinks it is permanently in sleep mode when hooked up to my computer. I have tested the monitor on another computer and it works fine. I also tried a different monitor on the same computer and had the same problem.

I was having some freezing/stalling/start-up issues a few weeks back and ran some hardware diagnostics in safe mode that said it was a faulty hard drive. I installed a new hard drive and the computer worked fine for a few days, now the monitor/computer is stuck in sleep mode. I have no idea how to run any sort of diagnostics, as I can't even tell if the computer is booting up because there is no display.

I tried re-seating all of the hardware components and that didn't work. I've tried to blindly get it into safe mode and that didn't work.

I don't think it's the new hard drive (I'm wondering if it was ever the old hard drive!) as it functioned great for a few days. My poor computer went to sleep and won't wake up.

Please help.

Thanks in advance,
Christina

I'm not sure how to check anything when I can't see a thing... It's just weird, it worked for days and then when it went to sleep the other day, it never woke up.

Could it be the PSU?

Yes, the pc is running. I can hear all the fans spinning, and it "working". The speakers are working, I can hear various normal startup chimes and tones that my computer is set to do.

The screen is completely blank at all times, other than displaying a "monitor is going to sleep" message.

I have checked another monitor on the computer, as well as hooking it up to my HDTV via a VGA cable, and had the same result. TV said no input signal, other monitor displayed the same "going to sleep" message.

I have tried two different cables and got the same result.

I checked both monitors on another computer and they worked fine - so it's not a brightness setting.

The graphics card draws power from the MB, no additional power cable. I tried re-seating everything on the motherboard and that didn't help.

I've tried disconnecting all un-necessary power draws and that doesn't do anything either.

As for a virus, I didn't even hook that computer up to the internet yet since installing the new hard drive.
I'm completely at a loss!

Thanks for the replies so far...

Okay... tried isolating each stick of RAM, still no dice. I triple-checked that everything is connected and firmly seated. I'm ready to toss this thing out the window.

also you say you added a new hard drive, when doing so did you add a new os and add the correct display driver?.