@scubamaggo:
As you have unfortunately found, chkdsk is not a data recovery utility, and using it can result in loss of data, depending on the state of the filesystem when it is run.

Getting a better diagnosis of the cause of the
original problem (after which you ran chkdsk), may be helpful. There are several posssible approaches from this point, but based on the limited info currently supplied, I just wanted to suggest that you make a raw clone (i.e. all sectors) of the existing disk onto additional storage before doing anything else, and don't do more (e.g. renaming files, as you suggested, or installing new software etc.) on the original disk. Run any recovery attempts against the clone. Making a clone will also tell you if there are any problems reading the original disk.
If the correct-sized chk files are not your NEF files, then some raw recovery (i.e. file carving)
might be more successful, e.g. using PhotoRec (or commercial software, of which there are many) but success depends on things like the amount of file fragmentation on the original disk.
Any DIY recovery attempts have risks (e.g. incorrect use of recovery software, incorrect techniques, any latent hardware problem becoming worse etc. etc.) so if you want to minimise those risks, you may want to consider getting professional help. That has to be your choice, based on your skill level, your experience, your assessment of the value of the data etc.