Have you locked yourself out of your Joomla installation? On simple solution is to simply click on the reset password option at the Joomla login screen.
This works in most cases, however if you also have lost access to the admin email address, your are in big trouble. Fortunately, there is a solution available if you are willing to get into editing the Joomla database. See bo-tutorials-and-howtos/general-questions/how-to-change-or-recover-joomla-administrator-password.html for these life-saving instructions.