I had a similar issue with the 17" monitor which I purchased to replace my dying display. My external display worked fine. The new display only showed the boot screen or target screen. At issue was the display not having the proper EDID Chip (or the chip not having the proper firmware for the MacBook Pro). When you order a replacement screen, you need to check with the seller (before purchase) to make certain that the screen in question will work for the MacBook Pro. You also need to replace with the same resolution, size, and backlight. LG Phillips made the original screens for apple. There are several companies which sell Samsung replacements (These have the power connector for the backlight in a different place and no screw-holes in the top for mounting to the bracket). If you get a Samsung or other OEM model display, it must have the EDID chip programmed for the MacBook Pro (before installation). I hope this answers your questions.