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!
The restoring of the old files alone won't solve the problem, because Joomla 1.0.13 auto-upgrades password storage for each user at first login after upgrade. So you may need to restore your old usertable (typically jos_user). Alternatively, you can search for ":" in the password column of jos_user to find the accounts which got auto-updated, and restore the old password value from your older backup. But, if you don't have that, you have some harder times ahead, but things can be solved!Ok, here you will find some help to reset access to your Joomla system:
Method 1: easiest:
Click lost password in front-end, enter your admin usernameand email (if you remember it) and check your email 
Method 2: well, a little less easier:
- go to databaseadmin (e.g. phpMyAdmin), and open database.
- then find table jos_users
- then find your admin entry (by search by username).
- there you will see a password looking like:
- 1023456789ABCDEF1023456789ABCDEF:1023456789ABCDEF (notice the ':').
- go to an online md5 generator like here:http://www.iwebtool.com/md5
- type-in a temporary password, and write down the md5 hash. (e.g. md5 Encryption for 'example' is: 1a79a4d60de6718e8e5b326e338ae533 - you could use this as temporary, but change it as soon as you can).
- edit the entry in SQL, and change password column of that entry to the one above for password 'example' or to the one you computed, or the old one from backup (notice: no ':' in it...)
Alternatively, you could copy the md5 password from another user that you register in frontend or of which you already know the password. Or, can use this tip for restoring the admin access to Joomla!
