Missing Header, Sidebar, Footer and WP Toolbar on Homepage

Site: https://www.dulwichonview.org.uk

I’ve been asked to maintain this website and its a custom theme created by someone else which seems to have been put together in a rather odd fashion.

I’m having a problem now whereby the homepage doesn’t display the header, sidebar or footer nor the new wordpress toolbar.

All other pages work fine but I can’t figure out where this problem is coming from.