L2 and X together on the mailbox brings up the mailbox cheats. I am pretty sure it doesn't affect challenges, I think it's only the cheat to get Spoot the Magic Llama that disables challenges etc. As the others have said, then choose Reset Sim and choose the name from the list. I never knew what Reset Sim was on console, I always presumed it reset them to whatever age they were when the game started. Then I had Sims starting to get stuck, thinking I'd have to delete the game, but the Reset Sim fixed it.
Sometimes, too, if you have more than one Sim, if you make a Sim go somewhere else in town, then come back, the stuck Sim will reset itself, too (not always, though).
