Let's say that you don't have access to your website anymore because it was hijacked. You need to change the password of your Super Administrator account, first you must know that the password md5 encripted so what you need to do next is to make a php file in your root folder of the website let's call it md5.php. Open an editor and put in the following line:
echo md5('123456'); save the file and close it.
Then open a browser and access the file:
What you see is the encrypted string for '123456' which will be your password. You can replace it with any other string, it's your choice.
Ok now we need to open phpmyadmin or any other database managing tool that you are familiar with. Select the database of you website and the browse the joomla users table called: 'yourprefix_users'. Search for your account and click on the edit icon. Now copy the encrypted string in the password field. Click save and now go and login in the backend.
Now please install a backup utility so you will never need the steps above and make sure you backup your site on a daily basis :).