I’ve been doing a fair amount of non-standard tweaking to Mountain Lion’s web services.
It’s not difficult to completely break the web server, and initially it looks like it’s near impossible to fix it unless you know exactly what you did to break it.
Fortunately, there’s a quick command you can run in the Terminal to reset the web services to their factory default settings.
Go into the Server app.
Turn off Profile Manager, Web and Wiki.
In Terminal, run the following command
sudo serveradmin command web:command=restoreFactorySettings
Then, go back in and turn on any services that you just turned off.
Be aware that this will completely wipe out any custom websites you may have defined – the entire web service is reset to factory defaults.