Denoise does nothing

I'm about halfway through a stage series I shot, and the denoise feature worked great for a while, but now it just does nothing. 

There's no error message, the other features continue working as expected, and the denoise process appears to do its usual thing, but the preview shows no change regardless of the settings, and the process indicator immediately jumps to 95 of 100 and never finishes the task.  I've waited hours for a single photo with no result (it previously took about 45 seconds to denoise a photo).  All the steps are the same; it's just not doing anything.

I've checked for updates, restarted my computer, and checked my storage settings.  The files are all .NEFs.  I can't think of any settings that have changed.  Thoughts?