I think it depends upon a few things, but I suppose that if you upgraded that some of your user directories may actually be 'locked' due to permissions error b/c you changed the computer SID even if you used the same login and PW combo as before.
I have found that happen more than once.....
Since I actually use a separate HD for my profiles, I have to manually add them to Fx upon a reinstall, and I have seen this more than once, though, oddly enough, not in my actual folder that houses my profiles - but YMMV with this advice.