Not only for DNS: your whole site is behind CloudFlares CDN.
Also, if your Apache is having issues (which is supported by the CloudFlare 521 "Web server is down" error), you should probably focus on your Apache first. Note that an expired certificate does not "block" Apache: Apache can very happily serve an invalid and expired certificate.
Please debug your Apache issue first, make sure CloudFlare can actually connect to your webserver and try again.