I'll assume you dropped and go from there.
Power off. Then disconnect the power pack.
Flip it over and loosen the 1 or 2 screws holding the ram cover down.
Open the door.
Prying open the two keeper clips on either side of the ram card, remove the ram.
Do the same for the other card if it has more than one.
DO NOT get them mixed up.
Reseat the bottom ram card at a 45 deg angle in the socket taking care to line the notch up with the alignment pin. Push the outer edge of the card down until the keepers click in to the notches in the sides of the card.
Do the same for the top ram card.
Replace ram card door and run down the screws.
Power up the system and let us know what happened.
Since the video subsystem uses the main ram, to me, this is the most likely culprit.