Assuming that you have absolutely no way to log into the database and overwrite the password, there is another harder way
On most debian-based linux distro's, when you install mysql, a certain user account, with a password, is created and stored somewhere on the filesystem. This account is used by the OS to do something I dont remember.
If you can find this username and password, you can use it to log into MySQL and overwrite the password.
Doesn't Joomla store the database credentials in some config file on the filesystem?