I've seen this problem before. Try doing a hard cache clear in your browser or switching to a different browser.
If that doesn't make the button reappear you may have to use a script. Do you have the Alfresco JavaScript console installed? If so, that's probably the easiest. If not, you should install it the next time you get a chance. Without the JavaScript console you can write a little web script to remove the user from the ACL.
If writing any kind of script does not appeal to you, another option is to install the Apache Chemistry Workbench. You can connect to the repository and adjust the ACL using that tool without code.
Or maybe someone else will suggest how to get the button back. But at least you've got some options.
