Changed ‘WordPress address’, now it’s broke!

The Options > General for my WordPress install were set to –

WordPress address (URI): https://www.mydomain.com/blog
Blog address (URI): https://www.mydomain.com

In a moment of madness (I was in “I wonder what happens if I do this” mode!) I changed them around to:

WordPress address (URI): https://www.mydomain.com
Blog address (URI): https://www.mydomain.com/blog

Now, I see all my content but with no CSS styles and if I try and login and change the setting back I get a 404 error when I access the Options > General page!

I’ve changed the siteurl in the options table of the database to https://www.mydomain.com/blog but this changes nothing.

Anyone know how to fix an idiot’s irresponsible dabblings?

Thanks