As far as I am aware "Monitor going to sleep" is not a message from your computer, but from your monitor itself.
This being the case, the problem would seem to be that the monitor is getting no signal, or a signal that it can't recognise.
You say this has happened since you moved the machine. It's possible that you've not correctly plugged in and tightened the securing screws for the monitor lead, or that the move has jolted the video card loose from it's seating.