Can you see the monitor when you go into setup? The sleep mode is controlled by the BIOS (enabled ACPI or disabled) firs, and then by XP. If it (video) never shows up in boot up (Basic VGA mode) then its likely a motherboard issue, possibly your video card slot (Is it PCI, AGP or PCI-e?) or something else. The latter two have their own special slot. Try using an old PCI video card if you have one or can borrow one and it should get you into the setup at least. With what you described it doesn't sound like a monitor or video card problem and probably not a "sleep" problem.
A few weeks ago my power supply fan quit on me. I noticed it almost right away. and limited my use on the computer to brief moments. As soon as I could I ran out and bought a new ps. After I installed it I could not get my monitor to come outta sleep mode so I took a look at my video card and noticed the fan on it was not working either. So I then went and bought a new video card. Now I have it installed and still the monitor will not come out of sleep mode. Prior to all of this there where a few times when I would have this problem but if I rebooted it would usually kick the monitor on. Everything turns on fans/harddrive/cdrom but monitor stays in sleep mode. I have tried a different monitor and the same results occur. also when i disconnect the monitor it gives me a message that there is no feed.
I run on windows xp if it helps