If you get 500 Internal Server Error in WordPress, you can check this tutorial.
If you're not using WordPress, you can follow these steps that will help you fix this error:
Step 1 - Enable PHP errors display
Step 2 - Analyse the result
Call to undefined function - change your PHP version, try all available versions. Do a code-review.
Parse error (Syntax Error) - check the file mentioned in the error to see if you can find any parse of syntax error.
Warning Access Denied - double-check database credentials, which you inserted in your configuration file.
Depreciated - lower your PHP version. You might need to try all versions before you find the one that suits your website.
If nothing changed and you still see the error 500 after enabling PHP errors display, try changing PHP version and enabling display_errors on different versions.
Step 3 - Backup restoration
You can also contact the Customer Success team for additional help if you are stuck on any step :)