It's a pretty common problem, you turn the TV/Monitor/Screen off for a few hours or more, and when you come back, you just have a black screen. You can hear the menu sounds, but the screen won't wake up. This seems to be a common problem for HDMI connected screens.
Hopefully this will work for you:
If your remote has a LiveTV button, hit it. It doesn't matter what channel it's on, the screen should now come out of sleep and start working as normal.
If you don't have a LiveTV button, try getting into the Media Center Guide using the Guide button. Then hit your OK button. Again, it doesn't matter what channel it's on. That should wake the screen up and start working normal again!
This works for me and for a few other people that I've suggested it to, hope it works for you as well.