Do you are an early adopter and you upgraded to Joomla 1.0.13? And you have Community Builder, VirtueMart or SMF Bridge...and you cant' login right now, and/or your users are complaining about the same problem. We're sorry for you... Do you have a backup? Then is simple, restore the Joomla 1.0.12 files, overwriting the actual files with those from the official distribution package and restore the jos_users or similar (or at least the passwords column from that).
Do you have backup, don't you? No??? Ouch... humm, then the job is a bit harder, but can be done!
So, still you can do the file restore? Good, do it. Then read below!