Ok, here some help to reset access to your joomla system:
Method 1: easiest:
click lost password in front-end, enter your admin username and email (if you remember it) and check your email
Method 2: well, a little less easier:
go to database admin (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:
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.
As Joomla Q&T WG member, I'm really sorry that Joomla core team rushed this release out without proper tests on final release, complete announcement and warning message.
Please express yourself on joomla forum regarding this. We can't do lots here, except working full-time on releasing a solid CB 1.1:
Upcomming CB 1.1 will be compatible with joomla 1.0.13.
Joomla 1.5 RC 1 is also incompatible with CB 1.0.2, same story...
Upcomming CB 1.1 will be compatible with joomla 1.5 RC 1 (we filed already 4 bugs to Joomla 1.5 RC1 this while adapting and working around).