Suppose you forget the password or username of magento
1.Just close your eyes for 2 minutes be relax
2.Go to magento database go to admin_user table and browse the values.
Just keep backup of password column value.
3.Now just copy below value
and replace with existing password column value.
This is nothing but md5(“admin123″);
4.Now give your username and password will be= admin123
Your Forget or mismatch password problem has been solved.
But not fully enjoy,
because after giving the correct password you may suffer to not able to login in admin panel.
So after this you can follow this step:
Read here : http://moonreturn.com/magento-admin-login-problem/