In this case I would recommend you to reinstall the program using the following link: https://www.nch.com.au/fastfox/ffsetup.exe
Before reinstalling the program, I recommend you to create a backup data file to avoid losing your information during the process. To do that, go to File > Backup Data > select a destination folder > click Backup.
Once you reinstall, you may need to restart your computer and try again.