The normal ways to recover a lost password are:
If neither of these is an option, then you have a third option, and that is using the occ command. occ is in the owncloud directory, for example /var/www/owncloud/occ. occ has a command for resetting all user passwords, user:resetpassword. It is best to run occ as the HTTP user, as in this example on Ubuntu Linux:
$ sudo -u www-data php /var/www/owncloud/occ user:resetpassword admin
Enter a new password:
Confirm the new password:
Successfully reset password for admin
If your ownCloud username is not admin, then substitute your ownCloud username.
You can find your HTTP user in your HTTP configuration file. These are the default Apache HTTP user:group on Linux distros:
See Using the occ Command to learn more about using the occ command.