If WampServer is showing an orange ‘Online’ icon in the system tray, but your localhost is not responding, there’s an easy fix – which can be hard to find, since in my case there weren’t any errors in the Apache system logs.
If this problem appeared suddenly (i.e. WampServer was working before), there’s probably something that’s preventing Apache from using it’s preferred port (80 by default). Skype is a notorious offender here – close Skype, and you can usually start WampServer. If you cannot find out / change the program using Port 80, go into the Apache conf file and change the LISTEN parameter to a free port other then 80.