1. Your mobo is not detecting only the GTX 960 or both the cards?
2. If you have a different system, install this card there and check if it is working or not.
3. You can also try to install it in a different pcie slot of you mobo.
4. If fans are spinning on 960 during startup?
Try these three options and it will tell you which is wrong. There is a chance that the GTX960 is dead.