how to repair your restore points. This is difficult to answer because you don' say how they may have been damaged or corrupted. It is possible that they are OK, but something is interferring with their use. Check out this site for some suggestions.
Can you go back six months? Well, that depends. How much HD space was allocated to system restore, and how many points were set? On this system I have it set for the maximum HD allowance, and right now I can go back six months -- barely. Had I, or the system, set more points, the answer would have been no.
Only you can answer this question. Go to system restore and go back to the earliest available point. The calender will answer. If it does not go back as far as you like, increase the disk space allocation, if possible. But this will obviously apply only to the future points.
Hope this helps a little.
Frank