Can't view admin pages – I am logged in but only the site front end is viewable

Wondering if anyone has seen this before?

So I can login fine. And the admin toolbar shows at the top of the screen. When I click on posts or dashboard or any other admin link the url displays correctly e.g https://mydomain/wp-admin/themes.php/

BUT only the site home page displays. I can’t actually see any of the admin pages.
(The front end of the site is working fine)

I’m thinking replacing all the files to do with admin pages might resolve it? Does anyone know which files to replace?

Thanks for any help