From B2evolution

If you have lost your password, follow this procedure:

1) Go to the login screen. If you don't know where it is, try yourblogurl/admin.php or yourblogurl/admin/

2) Click on the "Lost password" link on the lower right.

3) Enter your username. If you have forgotten your username, try "admin".

4) You should receive an email allowing you to log in. If you don't receive the email, CHECK YOUR SPAM MAIL folder for false positives. If you don't find it, check your other email accounts. If you still don't find it, check all your email accounts.

5) If nothing helps, you can find out your username, your email address or even reset your password by editing the database with PhpMySQLAdmin. Of course this requires full access to your webserver. See this forum thread for details.

If you do not understand those instructions, please contact your hosting provider's tech support and ask them to do this operation for you.

There is no additional magic trick that can be done to recover a lost password. You have to go with the instructions above.