If you have forgotten your username, add /index.php?option=com_user&view=remind to your domain name to find out what it is, eg
www.yourdomainname.com/index.php?option=com_user&view=remind
Obviously you'll have to use your own domain name
If you have forgotten your password, add /index.php?option=com_user&view=reset to your domain name to change your password - then follow the instructions.
www.yourdomainname.com/index.php?option=com_user&view=reset