Monitor going suddenly blank?

Had a strange on today, the ICT suite had been dismantled due to work being done in the suite area and we put it all back together last week. There had been no problems before the summer break, no work done on the PC's or server but now we have connected them back together and odd ones have now while a child was working on it the screen went blank, PC still on, power still on to the monitor but no response from keyboard or mouse, as though it had gone to sleep but could not seem to wake it up. Had to shut them down, loosing kids work and it booted up into windows as normal! Never had this before and not sure what it is or how to get around it? Any ideas?
Looked at the power settings on the PC and they are set to all being on always.:confused:

I can only suggest plugging in another working monitor the moment it happens to see if it's an issue with the monitor or the graphics output. Are these CRT's or TFT's, could be the backlight failing if they are TFT's.

They are TFTs. Can see the screen is on or has power to it not just from the light on the on button but that the screen changes from a deep black to a lighter black when the power is switched on. Cant replicate the problem on demand unfortunately so Ican see this being a frustrating one as I'm only at that school .5 days a week. Will have to check if its down to one make of screen or not as we have three makes in there.