Bought Wallpaper Engine on Steam but have to buy it again?

I bought Wallpaper Engine from Steam and haven't used it in a while, but I still had it as my wallpaper. I wanted to open it again to choose a different wallpaper and had to update it. After the update, I got a black screen. I waited a few days and tried again, but when I tried to open Wallpaper Engine in Steam, it was no longer in my library, and I have to buy it again to use it. How do I get Wallpaper Engine back without buying it again?

Thanks for the help in advance