You probably checked the settings, but first thing I thought of is the various window modes. Not sure if there are 2 versions or more. If the problem is new and you did no changes, this is probably not the reason. I play in full window mode (I think), and I believe this is the only mode where TAB will work. If you added CC recently, that might be the cause, although I guess you would have known if you added something to alter the window/UI etc. However, once my sim could not travel by phone because of a cc bed (!), so thee is not always a clear relevance.