was it on when you dropped it? Sounds to me like some data got damaged on the hard drive. go here:
Hiren's BootCD 9.7: Download Hiren's BootCD 9.7
and download the free hiren's boot cd. Burn the ISO to a disc and boot your laptop with it, then run one of the physical hard drive tests to make sure you didn't do anything worse than data damage. Assuming it comes up clean, you're probably gonna have to reinstall Vista.