Have you recently changed username or password of your OS Commerce installation? This error means most of the times that the username and password configuration of the admin section is not correct. This problem arises often because the confuration file of the admin section does not get updated automatically when you change any of the username and password of the OS Commerce. Result, a mismatch between the real username / password, and what is there in the config file of the admin section.
Solution: Open the two following files in your favorite text editor.
See at the end of these two files, you need to compare the values of DB_SERVER_USERNAME and DB_SERVER_PASSWORD. If your front end (catalog) is working, that will mean that these two information in your includes/configure.php file is correct, but the same in admin/includes/configure.php is not correct.
Correct the admin/includes/configure.php file, upload in the server and check. As always, please backup any original file you are changing for any error and future reference.