I had the same issue - no signal to my external monitor when hdmi was plugged in. I finally went in to "device manager" (click windows key, type "device manager" then open device manager. I believe I went to "display adapters" and clicked on the nvidia one, right-clicked and selected "properties" and found that nvidia was disabled (I guess intel took precedence?); enabled nvidia (I think it was under the 'general' tab), and immediately my external monitor came to life! Hope this helps some other people.