First, this is not a Laptop, but a desktop machine. I am running Precise Pangolin.
I often notice that, when I am away from my computer for a while (usually at least 15 minutes or so) and come back, the monitor will be black, and the monitor's power indicator will be orange rather than the usual green, so I just assume the computer is in suspend mode. But then I move my mouse, type on the keyboard, etc., and the display stays in the same state. The power indicator on the tower itself is solid green (On) all throughout this process.
Whenever this happens, I end up having to push and hold the computer's power button to turn off and restart the computer.
I have looked for similar questions, and the closest I can find are questions about laptops relating to suspend/hibernate mode. Does anyone know what could be going wrong?
EDIT (5/21/12): This is happening less often now, but it still happens. My temporary solution is to force the machine to suspend anytime I know I'll be away for more than 5 minutes. It isn't ideal, but at least it keeps me from having to "brute-force shutdown", and possibly lose a lot of data, quite as often.