My screen stopped working so I set about to replace it. I was able to connect an external monitor and used fn+F4 to toggle to it without any problem. I bought a replacement LCD on eBay and just installed it. However I can't get the new LCD to work, I've pressed fn+F4 several times to toggle back to the LCD but the display remains on the external monitor. Do I have a bum LCD or is there something else wrong? What is the resolution to this issue?