Sorry, missed that bit about being unable to access the disk.
I'd see if the disk can be accessed in another PC. If it can, your burner may need the lens cleaning. I'd also copy the contents of the other disks to the h/d (if possible) and experiment with erasing and burning.
Maybe easier to buy a new burner. They are very cheap here in the UK, probably moreso in the USA. For PC related stuff, $1=Â£1<g>.