500 error after upgrade

My client just upgraded his WordPress using automatic upgrade process, however, nearly all pages now get a 500 internal server error.

The site is https://www.harmonyyoga.co.uk/

The only pages that seem to work are the install page https://www.harmonyyoga.co.uk/wp-admin/install.php

and the upgrade page which tells me the database is up-to-date https://www.harmonyyoga.co.uk/wp-admin/upgrade.php

We have a backup of the database made before the upgrade so we haven’t lost any data. However I’m not sure what version of WordPress he upgraded from.

Is there a simple way of fixing this without reinstalling an old version of WordPress?

Thanks
Andy