If it's bigin when you try to acces to joomla administrator panel, Just a username and password problem !! You have just to update a jos_user in your joomla database.
Go to your joomla web site directory and open a configuration.php with bloc note or note pad to show what database name your joomla administrator site use. You have to find a line who have:
public $user = 'joomlauser251'; //MySQL username
In my case joomlauser251 is my DB name.
Login to your mysql:
mysql -uyourusername -pyourpassword
Select database for your joomla:
use joomlauser251;
Change password for admin:
UPDATE jos_users SET password=MD5(‘NewPassword’) WHERE username=’admin’;
And retry to acces again.
That’s all !!!