First of all, is that your ghost (does the IP match yours)? If so, and the network services support it, you can use the GHOST command to kill that connection so you can change to that nick. If it doesn't have the GHOST command, then you have only 2 options... wait until it expires, which could take a few minutes or longer, or else ask an ircop to kill it for you.
If that isn't your ghost (the IP does *not* match yours), then someone else is using that nick-- and apparently is authorized on it. Either you didn't register the nick and someone else took it and registered it (that means it's not theirs and you can't do anything about it), or else you let your registration of the nick expire by not using it for X number of days and someone else took it and registered it (again, you can't do anything about it at this point), or else someone found out your password for the nick and has logged in with it (in this case, talk to an ircop about it and they may be able to do something for you).