I had that happen to me. You can either a) Try reinstalling the whole game (which may not work, btw), or b) Find the nwn2cdkeye.ini file, cut and paste the information in it to another file like word or notepad, then delete nwn2cdkey.ini.
Reboot the game. It will tell you it’s invalid. Reboot your machine. Then, using notepad recreate nwn2cdkey.ini with the stored data. That may work. HOWEVER – no matter what you do, you will probably lose your gamespy account, and have to create a new one, with a new email account. That’s what I had to do.