Well, my hosting provider updated some of the webserver software on the back-end to the point that the site was inaccessible, thanks to the theme using obsolete coding. Whoops…
I’ve updated the site software to the latest version, and as such have had to change the theme in order to get it working again – the old theme seemingly doesn’t have a modern version, so this’ll have to do for now. I might change the theme later. At least the site’s working again!