I forget the timeframe, but after a period of time an unused grouped nick gets removed from another user's account. (Ex: I am rob77dp and group cokeandaspirin but do not 'use' cokeandaspirin for 30 days I now only have rob77dp and cokeanaspirin is released for other's use.). It could be this. Manual un-grouping can also be done by the nick/user that grabbed it in the first place.
Also, if you didn't have it registered, it is possible somebody registered (not just grouped) it. This poses a different problem of having to find this user/person and get them to unregister the nick.
The third possibility (I think you've been around enough to have already checked this though) is that you typed in a wrong password or didn't identify properly/soon enough...
--Rob
