1. Could be a cloud save that overwrote your local version of the world partially? (Removing inventory, but keeping map data, dungeon states etc.
2. Could be a bug (Never had that happen to me though, and I've played since it came out).

Now, a solution could be:
1. Grab all you and your friends can carry on your characters and create a new world, giving you a headstart on the new world.
2. Give up and wait a few weeks/months until you suddenly remember the fun you had with the game and decide to give it another go.