it sounds as though when she dropped it she may have knocked some things loose or damaged some components which unfortnately could also include the motherbaord.
If you dont feel particularly competant in this kind of thing yourself then please get an engineer to do it for you. theres no point in taking risks.
if on the other hand you do feel competant check on the following items:
BEFORE DOING ANYTHING THOUGH PLEASE GROUND YOURSELF. STATIC ENERGY IS BAD NEWS FOR PC COMPONENTS.
1. power and data cable connections from your motherboard to devices such as the hard disk drive and floppy disk drive etc.
2. Memory is seated properly in its slots. memory is probably one of the easiest things that can work loose from a motherboard.
3. Video/Graphics card. Ensure that this is seated properly in it rightful slot. If this works loose then no end of problems can occur especially your problem with the blank screen. Also check that any power cables etc are secured ok to the card and to the board.
4. check also for other damage to the motherboard such as cracks (you might not be able to see these) and connectors having worked themselves loose.
let me know if this helps