Mac Pro (10.11.5) can't log in, doesn't display screens
I figured out a way to solve this problem without too much trouble. However, it only works with a second mac computer.


Put the Mac Pro into target disk mode.
Use a cable to hook it up to another mac computer (I used a firewire 800 to thunderbolt cable)
Restart other mac computer and hold down option to see boot options.
Boot other mac computer from Mac Pro's startup disk.
Follow startup/login process as usual.
Restart both mac computers.


The Mac Pro now runs as usual.