After the battle against the Negaverse, the girls returned to their normal lives, without their memories of being Sailor Scouts. Then, Alan and Anne began using the Cardians to steal energy from people on earth, so Luna had to restore Serena’s memories. Serena became Sailor Moon again, so that she could battle the Cardians.
I hope there won’t be any more battles, but I must fight evil.
From Episode #147 “The Return of Sailor Moon”