I've had this kind of problem previously, and found, after looking through the event logs, it was usually because the graphics driver crashed while the monitor was in sleep mode. After much gnashing and annoyed noises, usually because I hadn't saved something I was doing in a program which did not have an autosave/autorecover function, I would bite the bullet and clean install the computer.
I did try graphics driver updates and backdates just in case, but found it seemed to be due to the crash while sleeping that the graphics driver never "restarted" like it should do, but it tended to not fix the issue. A clean install normally would correct this issue. I also found remoting to it and starting a new user session would sometimes allow it to sort itself out, but I decided that this was not a proper solution.