Error 503 Service Unavailable

Can anyone tell me please, what this error means, is it to do with varnish or ngnix? And how should I solve it - a step by step would be great (if anyone has the time), thanks.

I have:
19000MB Ram
Apache/2.2.27
PHP version: 7.1.0
Ngnix
Varnish
Firewall is on.
Mod security is not installed.
Varnish Disk Storage Size: 2G
OS is Centos 6.8

Whole thing is on Hyper-v

Thanks
Mark