Problem: Unable to log into Joomla administrator. The login form keeps returning back with the error You do not have access to the administrator section of this site.
Solutions: Quickly ask your hosting provider if there were recent server changes or upgrades. For a quick solution, you could use ftp or cPanel editor to edit the configuration.php file. What this would do is to allow you a temporary unsecure access to your admin area and then let you add a new super user to login with and change the admin password details.
At the end of the configuration.php file please add the following code and replacing 'myname' with the name of the admin super user you can remember the password.
Afterwards, you may log in and make the changes you want. Please do not forget to delete this code from the configuration.php file.
See more instructions here