If you are the owner of the website, you can check your FQDN (fully qualified domain name) is correctly resolving. Examining web server logs at the time of the error occurring will be a good place to start.
These monitor the web for any outages and allow users to report any problems they may be encountering.
Request blocked by a firewall: With cyber criminals finding more and more ways to breach corporate networks, firewalls continue to play a key role in stopping them in their tracks. This can just be a coincidence, or maybe driven by a big event, but it can also be a targeted DDoS attack. This is where the server has reached its memory capacity, often activated by an unusually high number of visitors trying to access the same website. Server overload: An overloaded server is one of the most common causes of a 502 error. However, if you’re feeling up to it and are curious to see if you can do something yourself, there are a number of different tactics that may work.