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.

[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!

[jrhoffman75]

Did you remove your lens and look in the mirror box?