This original error:
basically means that the MySQL process can’t start for some reason. As @Nick-B mentioned, it seems like the raw DB files weren’t writable:
Which isn’t a normal thing, and is usually antivirus locking things down, or possibly a file getting corrupted.
As you say, it’s hard to know exactly what caused the DB files from going into an unstable state. The best thing I can recommend is to get in the habit of stopping sites periodically so that Local can export the DB to a SQL dump located at Local Sites/sitename/app/sql/local.sql
Doing this will ensure that even if the raw DB does get corrupted, you can manually restore the site by zipping up the site and re-importing it.