Upgraded, now site vanished :-s

I run three separate wordpress installations on one domain (each under a different subdomain), and they all use the same user database – so my users can access all three sites with the same credentials/without having to re login.

My sites have been subject to some sort of hacker/malicious software, so I have had to upgrade.

I upgraded using the automatic upgrade button, and it did so successfully (so it reported)

I refreshed the page and everything went blank.

I can’t login in to it, but when I try, it takes me to an unstyled login form, then when I click to login, instead of executing “https://blog.mysite.com/wp-login.php” it for some reason goes to ‘https://www.mysite.com/wp-login.php” (which does not exist).

Also, the links on the unstyled login form all point to “www.mysite.com” rather than “blog.mysite.com”

Any help will be much appreciated ??

Thanks you.