Well I suppose the worst case may be that you have broken your hard drive.
Inside the hard drive are some small disks spinning round, and flying over these disks is a read head that is a fraction of an inch from the disk surface.
If you knock the hard drive sometimes the read head can crash into the disk surface making it unreadable (this is called a hard disk crash)
Here is a picture showing the disks, and the arm with the read head.
You may be able to see the stratches on the disk where the read head has crashed into the surface of the disk:
http://rodrigo.typepad.com/english/hard_2Ddisk _2Dcrash_2D29_small.jpg
If this has happened your disk may be unusable and you MAY have lost everything on it.
I am not saying this has happened but if your laptop cannot find the operating system on the hard drive it may be the case.
You could try and find a utility that tests hard disks and run that. It would need to be one that runs from a diskette if you can find such a thing.