Is there a service or method of repair for sandisk cruzer 4GB drives that aren't recognised by the computer

#1 try a the key on a different system or ensure a known good key works on this one, sometimes there are drive letter conflicts prevent the key from being assigned a drive letter.

#2 like system RAM and other memory type devices, there is no repair if the device is not functioning. They are pretty much a toss on fail item, costs more to repair then replace.