If you have a disabled admin account and no chance to use a second one (you should always create a second admin account ;)) and OCC is not working for you - then the best and easiest choice is the following:
1.) go to your config .php → search for ‘installed’ => true,
2.) edit into ‘installed’ => false,
3.) upload file via ftp into your config folder
4.) copy CAN_INSTALL into your config folder
5.) open your site
6.) have all you db data with you
7.) create a new admin account
8.) login and enable your first admin account
DONE!
I tried a couple of other solutions, but with my version (21.0.4.1) nothing else then my description worked.
Have fun see ya.
cube