[Moderator Note: Topic title amended. Your problem is no more urgent than anyone else's.]
I tried modifying my WordPress theme to integrate the "co-author" tags following the instructions on the page where it gives examples of how to modify the theme's author code. After click "update" under the section of code on the Editor page in the backend, everything went to white (as you can see when you visit the domain name) and EVEN THE LOGIN PAGE goes to that screen so now I can't even login to the backend to re-install the theme!
There were a lot of articles and high resolution images and content that I have already built-up for over a month now vested in this website. I need to be able to login and manage the website. We heavily rely on users to visit the site and now it has been down for almost 24 hours.