This is one of the secret cheevos and its called "Lost and found"
This is story related and therefore can not be missed.
This cheevo will be awarded upon completion of level 7 "The lost diadem"
This level sees you travelling through the room of requirement and occasionally battling draco and his gang. After you get the diadem you will have to run away from a big fire and after this the level will end and the cheevo will pop.