My thumbnails have disappeared from my saved files, help!

I downloaded the trial version of Corel 2021 and I didn't like it so I uninstalled it. When I went back to my Corel 2018 saved client folders and open them all my thumbnails of the files are gone and have been replaced by the 2020 Corel logo. It's a square with the pencil in it. The logo for Corel 18 is a circle. Does anyone how I can the thumbnails back? Some of my client files have up to 20 files and without the thumbnails I have to open each file to see what it is.