Mark III 5D

[KamdynFuller]

My shutter will not open when I try to look through my Viewfinder. Please help! I can take pictures through the screen but when I switch back to the view finder, it is black.

[Tronhard]

The likely cause is that the mirror is locked up, so it is blocking the pentaprism from displaying the image to the viewfinder, on the other hand the sensor is able to have an unobstructed view. 

What lenses were you using when this happened?

[KamdynFuller]

I was using the Canon EF 16-35 MM f/4L IS USM Ultra-Wide Zoom Lens

[jrhoffman75]

Check that you haven't inadvertently actuated LiveView.

[KamdynFuller]

It's in Live View Shooting, not Recording.

[jrhoffman75]

Switch it out of LiveView and you will get your optical viewfinder back. It’s an either/or. 

[KamdynFuller]

It sounds like the shutter opens when I switch from live view, but viewfinder is still black unfortunately.

[Tronhard]

A good point.  I took the comment to mean about switching back to viewfinder to mean that they had actually knowingly activated the switch.

KAMDYN: please check the diagram on P20 of your manual, shown below: it identifies a button, just to the right of the viewfinder labelled Live View Shooting Switch.  That switches between live view using the LCD at the back, and the optical viewfinder.  Try checking that is not rotated, and if so rotate it back to vertical.
  It identifies a button

[jrhoffman75]

In that case either mirror lockup is inadvertently set or your camera has a mechanical problem.

Go into the menu an reset all camera settings. See page 56 of your manual.

Wrench (yellow) menu page 4

[KamdynFuller]

I have already done that. I'm assuming it's a mechanical issue as well. Thanks for trying to help!