This happened to a friend of mine. She came to Spain as a student and had an NIE then when she returned to work here she couldn’t remember / find the number. Basically, she had to do the application procedure again at the oficina de extranjeria, with the photos, passport etc. and they found it on the computer or gave her another one - as she can’t remember the first she doesn’t know which! I would suggest you go to your local oficina de extranjeria and ask there, but probably they’ll tell you to fill in the form, take your photos and passport and do it all again.
Hope this helps.