Minecraft wiki entry for The_second_day, associated text: If your bed is "missing or obstructed" when you actually try to respawn, you'll lose your spawn point and go back to the default respawning behavior (random within 20 blocks of the world spawn point). This continues until you actually sleep in a bed again. If there is no place to stand (solid block, not glass, fences, slabs, etc.) when you try to respawn, your bed is considered "obstructed".