What camera produced the NEF file?
To open in Photoshop you need to have the specific version of Adobe Camera Raw (ACR) , or a later version that supports your camera.
However not all versions of ACR are compatible with older versions of Photoshop.
If you need a later version of ACR than will open with CS5 then your choice is to upgrade to CC or to use the latest free DNG convertor. You use this to convert your files then you open the DNGs (not the NEFs) in Photoshop. You can download the latest DNG convertor here (The "Downloads" links are half way down the page):
Thanks Dave, at last some useful info.
I am using Nikon D610. In my knowledge it released in 2013. Can you direct me which version of ACR would be supported by CS5 that can open the .NEF file created by NIKON D610?
Not want to opt for paying more money, i am student; CS5 is my limit. Hope that make sense.
Thanks once more time your help.
The D610 needs Adobe Camera Raw 8.3.. That in turn needs a minimum of CS6 (which is no longer for sale)
If you can't upgrade to CC then just download the DNG convertor from the third link above, use the convertor to convert your NEF files to DNG. Then Open the DNG file in CS5 with Camera Raw in CS5. The conversion will not affect the Raw image data.