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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Xbox 360 is currently experiencing issues. http://support.xbox.com/en-US/xbox-live-status Keep checking the status to know when it's all good.