LR won’t change your images to JPGs unless you Export them all, which would be more than a “I must have touched something” sort of process.
What is your evidence that your NEFs are now JPGs?
As was stated, Lightroom does not/cannot change an image from NEF to JPEG. What is the entire workflow you have gone through with this set of images?
sorry I do not know what the entire workflow means
I imported from camera memory card to lightroom 5
On my screen I see jpg but before (a couple of days ago it was reading NEF
Another note: the letters DSC * number is also something new that I have not seen recently
Either you have your camera set to shoot JPGs (or are using a mode like HDR that doesn’t produce raws, only JPGs) or the images are being run though some other program that is converting the NEFs to JPGs.
Check your camera settings, make sure you’ve set the camera to shoot RAW and then import one of these test files from the camera or camera memory card directly into LR. Also open the SD card in Finder (Mac) or Explorer (Win) and see what the files are on the card, itself.
I don't know about Nikons, but in some Canon models putting the camera shooting mode into Auto (Green Box) will cause it to make jpgs even though the menu setting is still on Raw. Might be happening here.
On my Canon, anything except P, Av, Tv, M, A-dep, is JPG-only, things like Full Auto, Portrait, Sports, Landscape, No-Flash Auto, etc.
Could it be that you were viewing imported NEFs before, but now you're viewing imported JPGs instead.
Since Lr can't change NEFs to JPGs, I postulate that either the NEFs are still there, or they were removed from your catalog (presumably, by you, perhaps inadvertently..).
Have you tried searching your entire catalog for NEF files?
e.g. library filter: text - filename ends with .NEF
(select 'All Photographs' in left panel of Library UI - in 'Catalog' section, near the top..).
PS - I'm assuming we're talking about previous imports. If new imports are JPG not NEF, then it's a camera thing, not a Lightroom thing..