ANSWER: There are 2 types of 500 errors: server based, or web page based. In 99% of cases, they are web page based with our customers. Our servers are extremely stable and almost never send out 500 (internal error) messages.
Therefore if you are seeing a 500 error and it doesn’t say (via Nginx server) then its from your WordPress site, 99% of the time cause by a theme or plugin. If you were recently updating or changing a theme or plugin, and suddenly saw the 500 error, so you know the cause was whichever theme/plugin you were dealing with.
To temporarily get your site back up you need to login via SFTP and rename the theme or plugin folder to something else. Then your site should come back up and you can login WP Admin also again.
To permanently fix the error you must determine which bit of code caused that to happen. If you are not a programmer, then you need to hire a developer to deal with this sort of code debugging as our team is not available to help with theme/plugin debugging issues.