If it cuts off whilst you are away from the pc (not typing or using the mouse ) then it's probably the screen saver. You can check your settings by right clicking on the desktop choose screen saver tab and look at the properties.
This assumes you have a pc running Win XP, other computers and operating systems will differ.
Next thing to do is to check to see if your VGA cable hasn't worked itself loose. See if it's plugged in fully at the back of the computer and at the monitor end.
look on Control Panel then on Power Options- it may be the setting for screen to switch off when keys haven't been pressed for 1 minute (mine is set at 10 minutes)