The solution below has been tested on Joomla 3.x.
Have you ever experienced the situation when you've worked several days on a Joomla site, then played a bit with the extremely powerful Joomla Access Control List (ACL) and figured out the next time you log in as a super admin you have no longer access to the admin menu or admin menu not showing up?
It's a very unpleasant frustrating situation. After searching forums for hours, here you go, I'll give you the solution!
The reason you're not seeing anything or a blank page in the Joomla admin when logged in as a super user it's because you probably played with the Special access level. To fix this you have no choice but to edit the table prefix_viewlevels on phpmyadmin. Edit the row associated with the ID 3 which should be named "Special" or whatever your renamed it and change the rules so that its value contain at least the number 8 which corresponds to super admin user group.
Your rules value should at least looks like , or [6,8] or [6,3,8,5]. Well you get it, number 8 should be included. Save your table, log out from Joomla admin, sign in again as a super admin, you should now enjoy the backend superadmin menus!