Graphics Card Issues

[DeWireFire]

So recently my girlfriend ran into a problem with her computer which I built for her where her computer would boot up but she wouldn't get anything to display on her monitor. No response from the monitor at all. Since I'm fairly decent with trouble shooting I took a look at it. The graphics card fans spin and everything so I ruled out (for the most part) that it could be a pci-e power cable problem. I've tried reseating it in the same slot and in the other slot to no avail. I would have used my current rigs graphics card but her power supply doesn't have two PCI-e 4x6s so I tried plugging it in to my rig to see if it was the card or the Mother Board at fault.
I did and still the same problem. Hence, I came to the conclusion that the card is more than likely dead. However, when I swapped my card back in I started having the same issue. I looked into it after fiddling with it after a little and I came across a forum saying if I was swapping cards I should uninstall my current video drivers before I swap the cards (which I didn't do). That could explain why her card didn't do anything in my rig but doesn't quite explain why my card won't work. Please help. :c

My card: XFX Double D FX-785A-CDFC Radeon HD 7850
Her Card: PowerColor AX6790 1GBD5-DH Radeon HD 6790

 

[Deemo13]

What are the rest of the specs for reference.

 

[DeWireFire]

What are the rest of the specs for reference.

Her Rig:

G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600
GIGABYTE GA-970A-UD3 AM3+ AMD 970
COOLER MASTER eXtreme Power Plus RS500-PCARD3-US 500W ATX12V v2.3
AMD FX-4170 Zambezi 4.2GHz (4.3GHz Turbo) Socket AM3+ 125W Quad-Core

My Rig:

CORSAIR HX Series HX850 850W
GIGABYTE GA-990FXA-UD7 AM3+ AMD 990FX
G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600
AMD FX-8320 Eight-Core 3.5GHz AM3+

I haven't had any issues with my rig until today. and I've been careful not to bust the card from ESD. My rig is fairly new only 6 months old and her rig is roughly 1 year old. Also if I've made any errors in my builds, except for the fact that the 850W PSU is overkill I was planning on installing a second card, please let me know as these are only my second and third builds respectively.

 

[Deemo13]

So when you plug the 6790 into either computer, are the motherboard graphics operational?

 

[DeWireFire]

So when you plug the 6790 into either computer, are the motherboard graphics operational?

Neither boards have on board graphics.

 

[Deemo13]

So, neither work currently?

 

[DeWireFire]

So, neither work currently?

No. I've even tried reseating the cards in a different slot and using different PCI-e cables as well as a mix of the two. I did this with both cards and nothing. I still have one old 9800gt that still works in my old rig (which I've been using), but I'm apprehensive because if that one stops working as a result of installing it then I'll be without a computer.

 

[DeWireFire]

I also have tried clearing the CMOS and still nothing. I want to say that both cards are dead but my card, the 7850, worked fine before I plugged the 6790 in. I'm just really confused as to what the problem could be.

 

[Deemo13]

Same here. A dead card wouldnt kill a motherboard.