Another thing that might have happened - the queen might have swarmed. It's not uncommon in the spring. Sometimes they don't like where they are living, sometimes there is a queen power struggle, sometimes it just happens for no reason.

I have heard that some beekeepers use a trick where they keep a spare empty box somewhere else on their property. If the queen wants to swarm, she'll often decide to move into the empty box. Then the beekeeper gets to keep a hive that would otherwise be gone.