Many networks don't allow you to register your nickname only your username (but people often never realise that a NickName and UserName are different) This is the case for networks like the Undernet and QuakeNet.
If you're absolutely certain that you registered your nickname, then it is very likely you're using DALnet or some other network with NickServ.
Try:
/NickServ Ghost Nickname Password (DALnet)
Or for other networks:
/msg NickServ Ghost Nickname Password
Also try your networks #Help channel, or type /list *Help* for more info.
Happy chatting.
Regards,