Suddenly the domain connection fails.

There are several potential reasons why your domain connection might suddenly fail after previously working. Here are some possibilities to consider:

1. DNS Propagation: If you recently made changes to your DNS settings, it can take up to 48 hours for these changes to fully propagate across the internet. During this time, some users may be able to access your site while others cannot.

2. Incorrect DNS Configuration: Double-check that your A record in Route 53 is correctly pointing to your EC2 instance's public IP address. Ensure there are no typos or errors in the configuration.

3. EC2 Security Group: Verify that your EC2 instance's security group allows incoming traffic on ports 80 (HTTP) and 443 (HTTPS) from all sources (0.0.0.0/0).

4. Name Server Issues: Ensure that the name servers listed in your domain registrar account match exactly with those provided by Route 53 for your hosted zone.

5. Domain Registration: If you recently transferred your domain to Route 53, make sure you've confirmed any verification emails from ICANN. A suspended domain won't resolve properly.

6. Multiple Hosted Zones: If you have multiple hosted zones with the same domain name, ensure you're updating the correct one that's associated with your domain registration.

7. Caching Issues: DNS resolvers may be caching old information. Try clearing your local DNS cache or use online DNS propagation checkers to verify if the issue is widespread.

8. Certificate Manager Verification: For the CNAME record verification issue with Certificate Manager, ensure you've added the CNAME record exactly as provided by AWS, including any trailing periods.

To troubleshoot:

1. Verify all DNS records in Route 53 are correct.
2. Check your domain's name servers using a WHOIS lookup tool.
3. Use online DNS propagation checkers to see if the issue is global or localized.
4. Test accessing your site using different networks and devices.
5. Review your EC2 instance's security group and network ACL settings.

If after checking these items the issue persists, you may want to contact AWS support for further assistance, especially if this is affecting multiple domains or services within your account.
Sources
My domain is unavailable on the internet - Amazon Route 53
I changed DNS settings, but they haven't taken effect - Amazon Route 53
Domain name not propagating from EC2 | AWS re:Post
my domain is not working with aws. | AWS re:Post