I've been using Avast for years but have never seen it hang so badly while updating ... something about 'debug' very near the end of the update process stopped everything and, 'This is embarrassing. GUI failed to load', AV service is not available'. Windows Defender, which I don't use, complaining about AV having been turned off.
Tried 'Repair' twice but it didn't work. Uninstalled and re-installed, which seemed to go well enough if a quite slow on Windows 10 64 bit. All is well now, I think.