I’ve been getting several occurrences of Critical error when attempting to access WordPress on this site and I’m not 100% sure exactly what is causing the issue – I suspect it’s the automatic upgrade breaking things but doing an in-place upgrade to WordPress after the issue occurs seems to fix the issue each time.
Initially I thought it was a recent upgrade to PHP on my host and several old plugins using invalid php code that was causing the site to fail, however renaming the plugins and the themes folder to .old did not fix the issue.
After downloading the latest version of WordPress and extracting the files over the top of the original install the site then became available. This then enabled me to login, rename the theme directory, rename plugins and everything continued to work.