Today, as always, I opened the web page. At first, there was nothing wrong with it, and then it suddenly appeared:

An error occurred.

Sorry, the page you are looking for is currently unavailable.
Please try again later.

If you are the system administrator of this resource then you should check the error log for details.
Faithfully yours, nginx.

Is it a familiar mistake?

Check out the error page of nginx.

This 50x. Html is found to be the error on the web page.

Then check the port
through the command: netstat – tnlp to view the opened TCP port.
Found that the 9000 port PHP is not on.
Then restart the PHP service, and the problem is solved.