What should I do if there is a 502 error on the website?

502 Bad Gateway
a) Whether there is any abnormality in the number of concurrent connections and processes of your server.
b)b) Whether your iis or php-cgi, php-fpm process is configured incorrectly, and whether the parameter settings need to be optimized.
c) Check your website program and database to see if there are problems such as slow database query.