I encountered the same incident as mentionned above by chris..
After Updating manually to the newest version, and restarting my computer right after, Avast did not launch. After 10 minutes, I checked on the programs running and found that Avast was running and taking quite a chunk of memory as well as Avast Installer and seemingly a number of programs that i don't usually see and may with little doubt be linked to Avast. I double clicked on the Avast icons with no result, and decided to restart anew, at which point the computer stuck on restarting, forcing me to manually shut down my computer and start it anew a few seconds later. To note that the computer did start normally thereafter and i was relieved to see Avast App launch with the Updates detail screen as it should have happened initially.
So far, Avast seem to run normally and i'm quite dumbfounded by the event, nonetheless, i decided to report it, like my fellow above. I don't know if the problem call for a repair of Avast or anything like it, and since i'm not knowledgeable enough on the matter i think i will it at that so far as i seems to be running normally.