I think I have sorted it by simply closing the program and opening NXD again.
Well, in short... NX D sucks. And in my opinion, it sucks HARD.
From what I can tell, NX D is a stripped down version of what was a fairly decent file browser/editing package: View NX 2. Suddenly, along comes NX D and surprise! Most, if not all, of the really good tools that were part and parcel of NX 2 are gone. POOF! Tools like high-pass filter, monochrome conversion, Photo Filters, and the Auto Retouch Brush, all gone. It was like "upgrading" to a View NX 2 version from five years ago. While other sillier software providers have stupidly been including new features and functionality with future releases, leave it to Nikon to turn this ridiculous scheme on its head by actually stripping out perfectly good functionality with each new release... Gosh! Thanks, Nikon!!
My best advice to you, assuming you really want to start editing your digital images seriously, is to suck it up and spend the money on Adobe CC and start learning Lr or Ps or both. If you just can't part with $10 a month for an Adobe CC subscription, there's always Corel PaintShop Pro (currently at version 8) for about $60 or $80 (it goes on sale a lot). Barring either of those applications get started with RAW Therapee or GIMP. These are two very well-established, open-source applications (meaning free) that have correspondingly well-established user-bases and support documentation. Simply put, if you want to do the right, you need the right tools. And in my opinion, NX D just ain't the right tool.