Hi,
Exactly the same here - on opening Avast, it told me that "Avast needs to restart your computer" (Which it does every now and then). On restarting, I opened Avast from the system tray icon, and hit the "Run Smart Scan" button. After about 1 second, the GUI disappeared, and the system tray icon went as well. I can start Avast from the Start Menu, but it will still do a disappearing act if I try to run Smart Scan. The Program Version is 19.8.2393 (build 19.8.4257.546), and I am running 64 bit Windows 10.
Hopefully this will be fixed soon - a virus scanner that doesn't scan is rather like having a chocolate teapot!
Cheers - Henry