[2019] How does this happen?

Hey there @Brizzy

Welcome to our community.
It is possible that you died while being offline. If your base is built in a hostile environment like the volcano, it’s possible you died out of environmental damage. Make sure to tuck in all your valuables in a nearby chest or container before logging out to prevent situations like this.
If it’s on a private server of your own, you can choose to not let offline characters stay in world to avoid this situation.