Hi,
If it's not being detected by your PC in Device Manager that means it can be a hardware problem.
- Try and reseat the graphics card from your PC, remove it from your PC for couple of seconds then put it back in making sure it's seated properly.
- If it doesn't work make sure also that the power cable is properly seated on the graphics card.
- Next is to try and install the latest graphics card driver from NVIDIA site.
- Here's the link: nvidia.com/download/index.aspx
- If all these will not work it's a hardware problem already.
You can also try these article on how to do a clean install of the graphics card driver.
http://www.tomshardware.com/faq/id-2767677/clean-graphics-driver-install-windows.html