Why are my NPCs disappearing, and chest loot too?

I Received a Dryad after killing the Eye of Cthulhu, but it’s now gone. There’s still proof that I killed the Eye of Cthulu. The chair that made the Dryad’s house is gone too! Also, my Goblin Tinkerer just disappeared too. And I still have the workshop. I have two chests at the top of my house which includes valuable things – everything inside it is gone! I am very confused. No other chests seem to have had anything happening too. What do I do?