Unfortunately, but there might not be a way for you to recover your Nintendo Network ID, as they are locked to the original Wii U console that they were created on.
And while you are not actually trying to link your original NNID on another Wii U console, whatever reason that deleted your NNID has also erased all the information that it was linked to your console, so it is like trying to re-link your NNID to another console.
So really the only thing you can do is call Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, and see if there is a way to recover your original NNID and also any of your eShop purchases you downloaded onto your Wii U console.
And also you will want to contact Nintendo to figure out exactly why your NNID was deleted, as if it is a problem with your Wii U console, you might need to have it repaired or the same thing might happen again and another NNID is mysteriously deleted.