Error 523 indicates that there is an issue in the origin web server and it has become unreachable.
Common causes
The most common causes for Error 523 are outlined below.
Changes in DNS settings
Sometimes host providers update the origin IP information for their customers. In that case, make sure the the new origin IP address for your A record is reflected in your Cloudflare DNS settings. Wait up to 5 minutes for the change to take effect.
Internet routing problems
Sometimes internet routing issues occur between Cloudflare's systems and the origin web server where the site is hosted.
In some cases, the holdup could be with an internet provider sitting between your origin and Cloudflare, or with the origin itself. If you are confident the problem isn't your web hosts, a useful piece of information to give to Cloudflare's support team is a traceroute between your origin and one of Cloudflare's IP addresses, ideally one of the addresses that usually sends requests to your site. See how to run a traceroute test.
Issues with Cloudflare Railgun
If you are the customer of a Cloudflare Hosting Partner and you are using their Railgun, you should contact them about this. If you are running your own Railgun, then you should contact Cloudflare Support and provide the most recent syslog file from your Railgun server.