Hi!
Thanks for all the help. I finally resolved the problem. So I’ll type out the resolution in case anyone wants to know. It turns out that I overwrote both index.php in the root directory AND index.php in wp-admin. So, to fix it, I uploaded those files using my ftp server. If you don’t have a copy of those files, you can find them by downloading the installation packages at https://www.ads-software.com/download/release-archive/, especially if you’ve got an older version.
Hope that helps.