Web server down. Error code 521

If I access the EC2 website directly without Cloudflare in between, will the page appear?

Was the website accessible in the first place?
If you were able to access the site, check to see if the web server in EC2 (for example, Apache) is running.

If you are not able to access it, please check if communication from Cloudflare is allowed in the EC2 security group, etc.
IP addresses from Cloudflare are listed at.
https://www.cloudflare.com/ips/

For additional troubleshooting, please check the following sites.
https://community.cloudflare.com/t/521-web-server-is-down-web/398712