Can't access login page after update

I just updated to whatever the most recent version of WP is, and when I go to mydomain.com/wp-admin, I get my template, with the following text in the body: “This is somewhat embarrassing, isn’t it?
It seems we can’t find what you are looking for. Use the search bar to find the content”. No login page for username/password shows up, and the URL becomes: https://mydomain.com/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F

I’m savvy enough to use my template when I’m in it and update things through there, but I do not know coding, and am clueless with FTP, or any alterations to PHP files and the like. Can anyone help with a layman’s explanation of how to fix this? My site is still running, but I can’t get in! Thanks.