World deleted/destroyed when optimized

I optimized my world which was previously in 1.15.2 to 1.16.1 and when I went back into it everything was gone and the world was completely different. My inventory was the same but my base/builds were all gone. I thought I had just moved so I went to the exact coordinates of my base and nothing was there, the terrain was all different. I researched it and it seems I followed the instructions and the optimization was not suppose to effect my world at all.