if (and only if) you have the correct information to identify with a nick on that network (meaning you're the legitimate user of that nick), but the nickname is already in use, you can always do a /nickserv ghost nick password in order to remove that user who is "illegitimately" using it. Then you can log in with your nick and your password... if that's not possible, it means that either someone discovered your password and changed it, or that the ircops did that job, and you'll have to look for another network/channel to log in...
