Hello,
A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Review the suggestions in this Community Tip for Quick Fix Ideas.
Basically, you’ll need to contact your hosting provider for clues. Ask them to allow connections from Cloudflare’s IPs, as those may have been rate-limited.
Also, even though you say you didn’t change anything on your site, sometimes hosting providers apply automatic updates (especially security updates) to WordPress and some plugins. Your wp-config.php may also be set to perform these automatic updates.
While keeping WP and plugins up-to-date is in general a good idea, this may at times cause strange behaviors. WordPress just did a security update in the past few days (to version 5.7.1).
Finally, you may want to search on Forums | WordPress.org to see if others are facing a similar issue.