happened to me. Went to my ortho and he said there are a couple of options
1) they can sort of file away some of the enamel either side of the tooth which has gone back, and they they put something infront of it. So it looks like ur teeth are now straight, but they actually arent
2) braces again
he gave me another retainer to wear at nights, and said i should go see him again when i'm ready. But u've got to be careful, because without the retainer, u teeth are likely to keep moving