While having the avast icon disappear and being unable to open the GUI can be concerning, I think you've hit upon an old IT tech trick, i.e., reboot when in doubt. Sometimes glitches happen, and a simple reboot or two fixes things.
Windows 10 lock screen will sometimes show garbled letters and words at times, especially after days of not rebooting. Sometimes logging in and out will fix that, or sometimes it requires a reboot. Nothing to worry about.
As long as these two do not reappear anytime soon, you should be good to go.
