If it is like the ones I have delt with in the past it is part of the window mount (part 19 in Craby's list) and it is not serviceable. Sometimes they can be welded back onto the mount but usually they will just break right back off. Get a new window from a wrecking yard and check to make sure the roller is in good shape as well as the area around the stud that holds the roller in place.

Massey