Left/Right Margins

[jazenbee]

One of our pets laid on our keyboard and now blackened 3" left and right margins show in all displays. 
The graphics drivers are current and the card's status is shown as working. 
Since it appears a key or combination of keys caused this, I'm not in favor of changing the resolution, messing with the registry or doing much of anything with 'Settings' changes (unless some combination of keyboard keys was the reason).

What keys could have caused this?

[strollin]

It's possible that a key-combination could have done that.  It's dependent upon what video card driver you have.  As an example, here are some instructions for key combos for a computer with Intel graphics drivers: https://www.intel.com/content/www/us/en/support/graphics-drivers/000005491.html

If your computer uses Intel graphics these instructions should help, otherwise you need to look up the instructions for whatever graphics driver your computer uses.