I'm afraid that's quite a generic error so I'm unable to say why it happened.
Could you please attach the following two files:
C:\ProgramData\AVAST Software\Avast\log\aswBoot.log
C:\ProgramData\AVAST Software\Avast\report\aswBoot.txt
I'm not saying they will necessarily show the cause, but they do contain some more detail, so they can help.
Thanks.