It sounds like she might have touched one of the controls on the monitor itself - on mine they are tucked away out of sighteound the side, and I forget where they are - in fact I had to have a look just now to remind myself.

So have a look round the bottom and the side and see if there are any knobs or buttons or whatever. If so one of them woudl be for turning up and down the brightness, like on a TV.

If you can't find anything give the details of what make and model computer and monitor it is here and some helpful soul will probabaly be able to suggest something.