Websites are going down

My Apache, NGINX, Varnish all are running. All websites are down with

Error 503 Backend fetch failed

Or

502 Bad Gateway
nginx/1.20.1

When I restart apache, websites start working and after sometime, all sites go down again.

Please help

This is solved.

Few days back I tried to increase the php timeout very high to accomodate an import.

I removed those custom config in php fpm.

Issue resolved.