What kind of back- end server is running your website? Often times we see this with CloudFlare IP's getting blocked by the servers firewall, so that very well could be what's going on. May want to reach out to your provider and make sure they aren't blocking CF IP ranges which can be found here: https://www.cloudflare.com/ips/
For this error, you need to check your web hosting is running or downed, because it related to host error. If you can ping to your IP address or your hosting then you need to check on Cloudflare if there are some wrong configurations there.