Building disappeared in Minecraft world?

I have my world in 1.8. Built. Then later without problems in 1.15. Opened and continued to build. After that, today, but again open in 1.8. When I later again in 1.8. The world was still there to play, but nothing more of what I had built. I only know that individual blocks are missing… Can I restore the building in the - now empty - world?

This is - usually - due to the new engine in Minecraft for worlds. Since version 1.8 still uses the old loading mechanism, it simply can't load the world. Before loading the world in 1.15, did you click the "Create backup" button or "I know what I'm doing"?

I clicked 'make backup' … But can't find a copy of my world in Minecraft or in the saves folder

You can find these backup copies in the Backups subfolder as a ZIP file. If you unpack it, you will receive your world folder. Just copy this into the saves folder. Then you can join the world again.

another screenshot.

The file that I find in the backups folder is named differently. Still tried the world of the same name, but was promptly another world. I don't find a zip file that is called my world. Now I'm very sad.