You're on the right track. When computers boot with black screens (no video or loss of video) the first things you want to check are the video card and the memory to see if either have failed.
I have a couple of other things you can try:
- Check to see if the CPU has worked itself loose. This will cause an instant loss of video.
- Check to see if the LED (known as a "dummy" light) on the motherboard comes on. If it does, then neither the power supply or the motherboard are part of the problem.
- Check to see if any cables attached to the case have been placed on the wrong jumpers. Since you said the video went out when your friend "bumped" the case, I dont think this will be the problem, but misplaced cables will also cause a loss of video. In order to check for the correct positions, you will need to have access to the motherboard's user manual.
- If all else fails, connect the same video card in a "known good" system and see what happens.
I had the same problem with NVidia GeForce 9500 cards going dead for no reason and taking down the system with it. Had to replace system memory in addition to video cards.
Also, take note of the beep codes that come out of the system speaker when you turn the computer on. Beep codes (also known as "POST codes" = Power On Self Test) should tell you where to start looking. The codes are specific for each computer manufacturer and motherboard, so make sure you check them based on the system maker and motherboard type.