See issue here as well: https://forum.fopnu.com/support/46

Here is what happened essentially:
"I had the exact same issue happen with Tixati (twice)- where it was automatically uninstalled and this week after running fopnu for nearly 6 months it was uninstalled upon a reboot automatically with no warning.

Fortunately when you re-install, the configurations are restored and not reset."

Temporary fix/solution: Reinstall the program

THIS WILL HAPPEN AGAIN! Until the developer root-causes it and applies a fix.

if windows defender is what removed the program you have to allow fopnu and/or tixati in the windows defender settings. microsoft/defender are being jerks about torrent clients and file sharing.