What problem is this?

There is a vertical red and teal (used to be blue) line on my laptop screen.

How it started (if it's relevant): One morning I forgot my laptop was on (the monitor was turned off and it runs very quietly) and packed it into my backpack. I don't know if anything banged into it while I was walking but when I took it out again there was a vertical blue line on it.

Now it looks like this:

 Spoiler

Possible problems:
Graphics card
Graphics card driver
The monitor

When I hook it up to another monitor, the lines didn't show.

 Spoiler

If it's displaying fine on one monitor, but not on the other my guess is the problem is with the monitor...?