Ok so first I will give you my specs on my PC and after, will explain the issue

GPU: Geforce GTX 780
CPU: Intel i5-4670k @ 3.40 Ghz
Memory: 16gb 
OS: Windows 10
MOBO: Asus Sabertooth z87

I built this PC and had everything running just fine. I had dual GTX 780's, identical cards except one is SuperClocked and the other is not. I had an SLI bridge attached and everything was great and dandy. My friend from work decided he was going to build a PC of his own but could not yet afford a decent graphics card so I let him borrow mine until his next paycheck. I removed my GPU and he had it for about 3 months or so until I finally got it back. It has sat around for a while because I only play low graphics games so I really didn't need it until I decided to put it back in. I installed everything along with the bridge and now my computer won't recognize the second card. It doesn't give me the option in Nvidia Control panel to enable SLI nor does the card show up in device manager. I have taken both cards out and placed both of them in the primary slot and they both work just fine, but it will not detect them together. I have updated my graphics card drivers, uninstalled them and reinstalled them, and checked to see if it was disabled in my BIOS but I never saw an option to disable or enable. PLEASE HELP!

I inserted my primary card in the secondary slot and tried running it and it came up as No Signal. But it's hard to believe my slot just randomly took a crap on me? It's less than a year old and has never been used or accessed since I swapped the card out last time. Is there anywhere in the BIOS that i'm missing that may have my secondary slot disabled by chance? Hopefully i'm not going to have to replace my MOBO already..... Thanks for your reply though. I really appreciate the help

I hit Reset to Default in the BIOS menu and i'm assuming that reset everything for me. No card still popping up. Weird to think that it could just go out without anything touching it or interacting with it. I guess I will be replacing my MOBO ASAP. I saw a program called Hyper SLI? Would that possibly be a quick fix for me or is that just outdated software. 

Alright. Thanks for the information man. Guess i'll be replacing the less than a year old MOBO 
++rep

Sajin
Sounds like your motherboard has a faulty pci-e slot. Test both cards in the secondary pci-e slot.
+1
 

shocker01134
I inserted my primary card in the secondary slot and tried running it and it came up as No Signal. But it's hard to believe my slot just randomly took a crap on me? It's less than a year old and has never been used or accessed since I swapped the card out last time. Is there anywhere in the BIOS that i'm missing that may have my secondary slot disabled by chance? Hopefully i'm not going to have to replace my MOBO already..... Thanks for your reply though. I really appreciate the help

Welcome to the form,
 
When hardware cannot be seen in Device Manager - it is a hardware failure issue
 
You can inspect your PCIe slot with a good light, hoping to find something stuck in the slot that may be causing your problem.

If you did not power down your PSU & remove the plug or failed to follow ESD (electrostatic discharge)
 
Yes, you could easily destroy the PCIe circuit. 
 
Most if not all MB warn that power still goes to the MB, even when in OFF mode. 
 
You must remove the plug and wait for the PSU capacitors to discharge, before adding or removing hardware.
 
Also ground yourself before touching the electronics