Try this. Shut down your mac and unplug any external devices. Then unlpug the mac for about 10 seconds. Plug everything back up and restart. Worth a try. Also, any 3rd party RAM used? It could also be an external device that is causing the problem. You can try to narrow that down by unplugging everything except the bare minimum and add a device one at a time until you get the fans grunning again. Thats all I got. Hope It helps.🙂
This happened to me once. The Apple Support Chat guy said that you need to check your power supply to make sure its tightly fitting, tape the thing in if you have to 🙂