Get someome to go into the server and kick you out, or the server host will do it remotely. If that doesn't work, like Rony said, the server needs to be reset and rebooted.
It happens when there is a failure/desync in the connection between the server host, LFS hosting system and the user's computer. You physically are booted or out of the server but virtually your account gets stuck in the server.
This is not to be linked with actual disconnection or latency issues direct between the user and the LFS. that just causes your normal DC or JOOS.
This is at least what I know and think is happening. I may be just talking shit lol