If another user is using your nick and you haven't turned nick protect on, you can use this command to disconnect the user and recover your nick. After using the recover command, you need to use the release command or wait two minutes to get the nick back. Services puts up an enforcer, to prevent someone from auto-rejoining with your nick. If you have identified for the nick already, then you do not need to include the password. If you have not identified, then it will not work without the nick's pass.
/msg nickserv recover jimmy jimpassword
Recover is a harsh command in that it disconnects the person from the chat network. You can try asking someone to stop using your nick before recovering it. But if your nick is in use and you want it back, this is always available.