I don't know how related this is: MS recently put out an update that blocks workbooks that were downloaded (unless they're in a trusted location). This update affects files that you PREVIOUSLY had working just fine... There is no warning, basically, suddenly addins no longer have their tabs, even if the add-in is shown as installed.
Fix: find the file, right-click, unblock it.
I spent a bit of time troubleshooting this with a client yesterday...
I know that they've blocked files that were downloaded for quite some time.. but this update seems to have gone back in time to older files now...