Updating page after editing, now it wants me to install WP

I was just editing my site’s ‘About Us’ page. I’d edited it, updated, viewed the changes a couple of times. Then in my most recent edit, I clicked update and was taken to the WordPress Installation screen (url=http://www.mysiteurl/wp-admin/install.php)

I cannot do anything in the backend – it just takes me to the installation page. I’ve tried viewing the site in another browser where I’m not logged in, and it shows the installation page.

I’m worried if I go ahead with a re-install, it’ll overwrite everything. Yes I do have a backup, but what’s happened to cause this and how do I remedy it?