Menu items missing in WP Dashboard

I built this site – energyalton.org.uk – using Underscores as a base. It’s been running with no problems for a few months. It then went down. I disabled all the plugins and the site went back up. I then enabled plugins one by one to ascertain which of them had caused the problem. Weirdly I was able to reactivate all the plugins.

The only issue I now have is that the Appearance menu in the dashboard is incomplete. I can only see Customise, Menus, Header and Background, but Themes, Widgets and Editor are missing.

The site uses Widgets so it’s crucial I and the client have access to these options.