Gigapixel re-install not working

After taking the new version of Gigapixel for a test drive, I decided to stay with my ‘owned’ version 5.8. It is installed, and I’ve also downloaded and re-installed this version but it’s not working. Trying to open “C:\Program Files\Topaz Labs LLC\Topaz Gigapixel AI\Topaz Gigapixel AI.exe” does nothing. It doesn’t show up in Task Manager even briefly. What might I be missing or doing wrong?