Minecraft building disappeared?

I'm currently building a city in minecraft and have always played with 1.15.2. Today I wanted to try the 20w18a version, everything worked when I was in my world.

After a while I wanted to play again on 1.15.2. So I went to my world but all the buildings are gone and I can only see the shadows of the lights, and before I went into the 20w18a version I made a backup, but I don't understand where to find them.

Can someone explain to me how I can get my city back? I worked on it for months.

The copied world should be the 2nd in your world list. At snapshots I can only recommend making 2 or even 3 copies