Linux Mint 17.2 will not boot after graphics card swap with Nvidia and ATI cards.
 

Two days ago I decided to test my ATI Radeon card for its performance. After I tested it, I put back my normal graphics card that is a Nvidia GeForce card. Then the OS will not boot unless the ATI card is in. I have tried to reconfigure the X11 settings, but that did nothing. I do not have the error message, its just a black screen.

My main video card:
ASUS Strix Nvidia GeForce GTX 970

The one is tested:
I was unable to find mine, but has 821C2 on the side

Other:
AMD FX-8350
8GB Ram
64-Bit

Can you see anything at all? Grub or any post boot messages or can you access bios/uefi?

I have GRUB poop up, but no boot messages. And I can access the bios.

I can boot with nomodeset.

Although it is limiting my resolution to 720p

...so when you say "won't boot" you really mean "no graphical desktop".

seems mint automatically started using the other driver, but isn't able to switch back.

put your nvidia in, boot into nomodeset or recovery or a text interface, and go through the standard procedure to install an nvidia driver manually.

certainly described on linuxmint wiki/docs/forums, and if not there then definitely on ubuntu's pages.