When that happens and you're ripping your hair out, load the default render settings, this time don't go changing stuff all willy nilly, only what you know you need. Need GI? check on GI and nothing else. GI not showing? Check the photon intensity on your light(s) and make sure they're strong enough to actually travel from the light, make it to the objects and atleast do a couple bounces, so that means TEST RENDERS AND TWEAKS YAY!:p