Do you have access to the MySQL backend through phpmyadmin?
If you do, export the database to a text document. Once you've done that, drop all database's for that particular site. So everything is gone and it's an empty database with no tables, then import the text document so that it recreates all the tables. Then test access, if it's still not granted, clear the user table (Don't drop it) and re-enter your admin details for the site and then test again. It should then give you access again as an admin, once you've done that you may have to manually enter all the user details again (Though you can just import the user table minus the admin login details).