Hard drive not mounting consistantly? Data not accessible? STOP everything immediately and turn off your computer. If your hard drive is damaged, or it's directory corrupted, you could do further harm by continuing to use it, as recoverable data can possibly get written over.
If you are having a data access problem, chances are the data is recoverable. It could be a small (inexpensive) problem, or it could be a large (expensive) problem. We need to take a look at your hard drive in order to be able to make an accurate determination. Stop using the drive, and call us before it's too late...