Can you try your monitor on your brothers tower unit and his monitor on your tower unit? This will tell you if the monitor is at fault.
It may be that the PC is stuck in "Power save mode" but I doubt this is the answer. Has your brother changed any settings at all? Make sure he tells you of even the slightest change to the resolution or refresh rate of the monitor.
Has he loaded any games or programs of any sort just before this happened? Please post back to this thread with these questions answered and one of the forum members will soon sort the problem for you.
PS: please don't try the restore CD's yet until someone has suggested other ways to try and fix the problem. It may just be something very straightforward to cure.