Deleted all progress in world

I created a Minecraft map, so I booted up my Minecraft to finish it up, but it was still in version 1.12.2. I wanted to update it to the latest version of Minecraft, and when I went to enter the world on 1.13 it said "May produce minor bugs. Save a backup?" And I didn't because I thought the bugs would only create a little lag. And it was pretty laggy, so I switched it back to 1.12.2. When I reopened up the world, I found out those "little bugs" deleting ALL of my progress! I don't know how to get it back. Please, either fix the bugs, or give more information on how the bugs might delete months of progress.