If your password has been changed, here is a solution to get another one.
1- Upload another wordpress in a new directory
2- Create a new DB or a write another prefix in the config.php (let's say temp_)
3- Proceed with the setup in you new directory. Note the admin password that is given to you.
4- Log into your DB with PHPMyadmin... Now you have two database, or a big one with 2 different prefix wp_ and temp_
5- Browse to the new DB and find tmp_users (if you choose a new prefix) or wp_users (if it's a new DB)... There you will find a user named "admin" with an encrypted password.
6- Copy this password then browse to your old DB (or old prefix) and paste the encrypted password to the old "admin" user...
With this solution, you should be able to connect to your old wordpress installation using the new password that was given to you with the new installation...
You said that you have a backup... I don't know if you have a backup of your database... If you have one:
1) Log into php myadmin
2) export your old database first
3) drop all the tables in your wordpress database
4) import your backup
That should restore the installation with your old admin password. Of course, you will loose all the stuff you wrote since you made the backup.