Who knows what went wrong.
There are so many 0's and 1's in a application.
If only one 0 thinks he is a 1 things go wrong.
It is not possible to found out what went wrong since a lot of avast (and other) files have changed.
Just be happy that it is working again and keep an eye on it to if it keeps working.
I don't know if it will help you, but some people have a list of things to do/check e.g. weekly or monthly with/on their system.
Some things on such a list can be :
- Check if av is working and up-to-date
- Remove all temporary (cached) files
- Check for application updates (not all will automatically tell you if there is a update)