Ughhh, some dude a while back did the same thing.....
Casual users and NTFS ACLs.........
He somehow was able to change it all back though. I have no idea how he gain access to the tools necessary to get back to the Security tab (if you are denied access to the whole disk, it's likely you are denied access to the tools necessary to change the permissions).
The hidden admin account is in the same boat as all other admin accounts.
If you can get back to the Security tab somehow, you'll need to seize ownership of the drive. That's the only way you'll be able to change everything back.