I usually put the drive in an external USB enclosure when this kind of thing happens. Plug it into another computer and copy your data off of it (make sure you get it all).
After that, run a couple of disk diagnostics (
Link Removed then
SeaTools) to verify that there's nothing wrong with the drive.
At this point, put the drive back in the computer and try a
repair install. If that doesn't work, a fresh installation will.