The default user name for a Joomla "super-administrator" is "admin."  But, if that user name has been changed, or you just forgot what your user name is, here is a quick way to get it back. Go to your webspace cPanel, find the phpMyAdmin database administrator, and open that.  Find your Joomla database in the list of databases (if you have more than one), and click it. 

Now, find the jos_users table and click on that name. 

Now, click on "Browse" so you can see all of the user records.

This list will show you the user names.  That ought to remind you want the user name is.  If you need to change the password, you can change that here as well. 

Click on the icon for the record that you want to change.

On the next screen, set the datatype to "MD5" and then enter your new password.

Click at the bottom of the screen to save your changes and you are done!