If you have become disconnected, but your client is still online, you can use the ghost command to disconnect the client. You must either have identified for the nick with the client sending the ghost command or you must include the password in the command.
Example:
/msg nickserv ghost webmaster webpass
When a computer disconnects abruptly, for example by crashing, sometimes it does not log off of the chat network. The server will eventually realize that the client is dead, but you may log back on and still see a "ghost" of yourself. This command is designed to allow you to remove these ghosts. You can only ghost a nick if you have registered it. If you have an unregistered ghost, just wait for it to disconnect itself. If it is causing problems, you can ask a network operator to "kill" it.