Hi As per your issue, please check the available free RAM in your instance when your website stop working, also the the inode in your ec2 machine. If both things are ok, then run curl -I [your domain name] from your local system and also run this command from ec2 instances. may be possible your website stop working in your particular network. If both output was same and status code is 200 then try to change your instance class to r6i.large or r5.large.
Hope these troubleshooting steps will help you to resolve this issue.
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