504 bad gate way. It's crazy

I left my laptop three days ago. I don’t change anything on my server, even don’t access it. And My website gets 504 bad gateway today. What the hell?


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To help, we’ll need more info:

Apache2, or NGINX?

Either way, you’ll need to hop on the server, and grab logs from your web daemons log files (/var/log/apache2 for Ubuntu/Debian Apache2, /var/log/httpd for CentOS/Fedora Apache2, and /var/log/nginx/ for NGINX)

Once you have the logs, look for errors. They will tell you why the site is failing. If unsure, post the logs (just the errors) here.