Blank wordpress pages after accidental ftp upload

Hi

Right, yesterday the site https://www.lewesbonfirecelebrations.com was running fine, no issues whatsoever.
This evening I was rushing about and accidentally uploaded an old version of the site via ftp to the same folder as the wordpress site. It most definately overwrote the index.php and added a folder (called assets) to the public_html directory.

I removed the assets folder as all my css/images etc are stored in the themes folder anyways but I appear to be left with blank pages and can’t even log in to the admin area at all.

What have I done? More to the point is this fixable?

Simon