Localhost refuses to connect after upgrading to Sierra OS

I solved this by commenting out this line from my httpd.conf:

LoadModule unique_id_module modules/mod_unique_id.so

I'm not very familiar with mod_unique_id, but it doesn't seem critical to have on my local development server (your situation might be different).

unique_id_module

This module provides a magic token for each request which is guaranteed to be unique across "all" requests under very specific conditions. The unique identifier is even unique across multiple machines in a properly configured cluster of machines. The environment variable UNIQUE_ID is set to the identifier for each request. Unique identifiers are useful for various reasons which are beyond the scope of this document.

After updating to macOS Sierra, Apache httpd moves the old httpd.conf to httpd.conf.pre-update and place the new one. In order to work the localhost work please try the below procedure:

cd /etc/apache2/  
sudo mv httpd.conf httpd.conf-afterupdate  
sudo mv httpd.conf.pre-update httpd.conf  
sudo apachectl configtest  
sudo apachectl restart  

Then test:

curl -I -L localhost

I had the same problem. I ran sudo apachectl configtest and got the message:

AH00526: Syntax error on line 6 of /private/etc/apache2/other/default-ssl.conf:

Invalid command 'SSLEngine', perhaps misspelled or defined by a module not included in the server configuration

I just renamed the default ssl config file so it's not to be loaded anymore. A apache restart later, everything came back.

I had same issue and i solved commenting mod_proxy_hckeck in httpd.conf

LoadModule proxy_hcheck_module libexec/apache2/mod_proxy_hcheck.so

EDITED: I found in apache.org that this module requires mod_watchdog, but this module is commented, so just comment mod_proxy_hcheck or uncomment mod_watchdog LoadModule watchdog_module libexec/apache2/mod_watchdog.so

hope it works!