If you have 2 or more GPUs and there is no 'SLI Configuration' in the Nvidia Control Panel under 'Set SLI and PhysX' configuration, this could be caused by a number of different problems.

First double check in Device Manager, to verify that both cards are showing up properly. It should see each GPU and properly recognize each with no error codes.

The most common causes would be; Drivers, SLI bridge, a GPU problem, motherboard or PSU problem. First, verify that your motherboard does in fact support SLI, it should specify this in the features. Double check your PCI-E slot configuration for your motherboard and ensure the cards are in the appropriate 16x/8x slots. Make sure your motherboard BIOS and chipset drivers are up to date. Also try a "Clean Install" of the latest Nvidia drivers, choosing a Custom Install and Clean Install.

If the issue persists, power the system down and try reseating both of the cards, double checking to make sure they are properly seated as well as double checking the power connections. Also reseat the SLI bridge to ensure this is fully seated. If possible, test with another SLI bridge to rule that out as a possibility.

If the problem still persists it would be recommended to test each card individually to verify whether they are both working individually or not.