It means that someone else is using your key, make sure you haven't given anyone your key.
No it doesn't.
Usually it does but there are other possibilities, he could have failed to connect to the internet (lost connection) and the master server authenticated him he could not authenticate from not being connected thus making everyone crash. This has happened to me before and it was internet loss.
no one can join back.
Two people can be on the same key at the same time from way different locations.
The only thing that happens to them is Badspot revokes their key after he finds out.