Macbook Pro 13" with Retina Display: Black Screen So this is not my computer, but my girlfriends. She was doing thesis work in the library when all of a sudden the screen to her Macbook began to malfunction. At the point it is at now, whenever it is booted up, it makes the normal booting sound yet the screen is completely black with gray bars in the center. The bars span the screen vertically, but are compacted into an almost solid gray bar horizontally.

The first repair shop nearby claimed that it was a broken screen that needed replacing, but I would like to see if anyone else has ideas before accepting that solution. Could it also possibly be issues with the integrated graphics card? Has anyone else had this issue?

And as a less technical issue, what would be required to use an external display with a Macbook Pro?
There is a small possibility that the video cable is mis-seated and simply unplugging it and re-plugging it in might help.  ifixit.com can help you with the take-apart.  It is not difficult, just make sure you ground yourself and disconnect the battery before touching anything.

Has the machine been dropped?
Has anything been spilled on it?
Is the LCD panel physically broken?

To connect a Macbook Pro to an external display you simply need the proper video adapter.  Either Mini-Misplayport to VGA or Mini-DisplayPort to DVI, depending on the monitor.  Most monitors have many options, including both VGA and DVI.

If the laptop and monitor are new enough you can use an HDMI cable.

To further diagnose your girlfriend's broken laptop connect it to an external monitor.  If the lines appear on the external monitor then the issue is the Logic Board.  If it works fine then you have a LVDS cable or LCD Panel issue and will need a new display clamshell.  iResQ.com does cheap display repairs.  No, I don't work for them. :)