First thing to do is to recover your data which is the most important matter of all. You can do this by removing the drive and putting it in another computer as a secondary drive. If the drive is unreadable as a secondary drive, then the partition table has been damaged, probably by bad sectors in the hard disk. Try to do a chkdsk operation on the damaged drive. Hopefully there are enough good sectors to relocate the bad sectors and the drive will be usable again. If the drive is really dying and chkdsk cannot make the drive readable you can attempt to recover your data using data recovery software. Scout the internet for good ones. Here google is your friend. Don't worry too much, there are worse things in life. Good luck!