I could be (as suggested above) that your dad has moved the box to install the monitor and doing so has unseated the memory.
Open up the case and check that all of the cards inside (memory, graphics, etc) are seated correctly. Might be worth taking them out (one at a time) and putting them back in just to be safe.
Check to see if your dad has installed any new software since you gave it back to him (including drivers for hardware).
to Cable Forum by the way
