Thank you for all the suggestions. I downloaded the viewer, then told the files to only open with NWC 2.75a.2. And for some reason they now open. Maybe the viewer did make some necessary changes in the registry? I should mention that Windows 11 had not updated recently, so was unrelated to that, and the files (and shortcut song links on my desktop) were working fine until earlier today. So I don't understand it, but that's OK with me - the song files are opening again. :-)
My NWC song files are not opening properly. When I click on a song file, it opens up the NWC program but not the song itself - it is just blank. I then have to click on the Open File icon, search for and click on the song file, and then it opens. Up until now, when I click on a song file it opens the song file directly, as it should. Has anyone else had this problem, or know a solution?