I was suddenly disconnected from a party and couldn't join back. It wouldn't let me every time I tried. When I tried to join a session it wouldn't let me. I turned off my Xbox now I can't get on Xbox live at all now, not even from smart glass.
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XBL is still having issues and they have all platforms now affected. If XBL is the issue, checking their support site (not here) is usually a better option.