I hope the following explanations are valid for the locking, too.
Generally there are at least 3 possibilities to save customization in ArcMap. The first is to save them in the MXD itself. Therefore you must check 'save all customization to document'. Due to the fact that all of your MXDs are locked I would guess you did not check this option. The second one is that ArcMap automatically saves settings in the so called normal.mxt. I believe that this had happened in your case. The good thing is that you can remove or better rename the file. ArcMap then loads the standard settings at startup and creates this file new. Please note that ALL your customizations are reseted (including the locking). If this was not your solution just replace the newly created normal.mxt with your backuped one. The third possibility is storing settings in the registry. In this case, I can't help you for the moment.
The normal.mxt can be found in the following location. Maybe you have to show hidden files in your folder settings: C:\Users\'username'\AppData\Roaming\ESRI\Desktop10.X\ArcMap\Templates\Normal.mxt