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?

Are you playing on mobile? If you are and you don't have auto-save on then you have to use "Save and exit" which is in the same menu as the beastiary and achievements. If you just force-exit the game your changes will not save.