WP-Admin Keeps Disappearing

We have a website that has had no plugin or theme updates for over a week and has been working completely perfectly. One morning we suddenly find we can’t access the wp-admin even though the rest of the site is continuing to work. There’s no error messages just a blank page.

I’ve tried removing all the plugins/themes but that doesn’t resolve it. I don’t really want to do a restore if I can help it because there is a lot of scheduled content in there that is till being posted and we’ll lose if we restore the database.

Does anyone have any suggestions on how to fix this?