Trapped in a Coffin

Once out, Jon was able to see what he was trapped in. It was coffin! "What the hell?" Jon said aloud. "Is this someone's idea of a sick joke?" He looked around and saw that he was in small room with four stone walls. Given that he had just gotten out of a coffin, he assumed that this was some sort of crypt or mausoleum. But how did he get in here?