Apache not running correctly after upgrading to High Sierra
Take a look at your hosts file, you should have entries in there for your localhost and local configuration. The hosts file is in /etc, so the best way to open it is by using Terminal and this (read-only) command:

open -a TextEdit /etc/hosts


You should see an one or all of these for localhost there like:

127.0.0.1   localhost   
::1         localhost
fe80::1%lo0 localhost


127.0.0.1 is your machine (aka localhost).

You can add other IP addresses from your local network to route domain names to a local development folder to the hosts file. For example, mine has custom entries from old projects like:

10.0.10.111     teambox.dongguk.ac.kr


To make changes to the hosts file, you'll need to use: 

sudo open -a TextEdit /etc/hosts


Enter your user account password when prompted.

Here is a full article on making edits to your hosts file: The Mac Hosts File: How to Modify /etc/hosts in OS X with TextEdit