I made some simple changes to the Nigeria Line up with the creation centre.
Now, my team is full of only 3 players: many Yobo's and some other guy.

How do I restore it to the previous state?

Copy "fifa.db" form FIFA CD #1 to "data\cmn\" in your FIFA install folder; then, copy the language file (eg. "eng.db" if you use english, "fre.db" if you use french) from the CD to "data\cmn\fe" in your FIFA install folder. If necessary, go to the "Load/Save" menu and load "Default Squads".
P.S.: By doing that you'll lose all your previous team management changes and erase any editing you've done.