You probably cannot restore it, based on how you describe the problem. It's more likely that you will need to reformat it.
Try booting the computer to a Live CD such as Knoppix
and seeing whether you can mount the partitions on your hard drive and take a look at them. This should give you a idea of the extent of the damage and also allow you to make copies of any crucial data files that are still readable.
Knoppix is not specifically designed for data recovery, though it can be used for that.
Here is a list
of data recovery tools that I found.