First, let's get your library back to where it was with a Library Restore. MC makes automatic library backups, so you can get back to a known state in case weird stuff happens. Weird stuff like this.
File > Library > Restore Library > (pick file here)
You should see lots of files from the last weeks and months with dates in their names. Choose one from before thi started happening. After the restore is complete, you library should look just like it looked before.
Second, your library might be going weird because of an MC setting. It almost sounds like your NAS path to each file is changing for some reason. You can get some more info by hovering over one of the files that won't play and looking at the file location that pops up in the information box. Or go to Audio > Files and look at the location in the FileName column.
There's a related setting in MC which might have something to do with this, but I'm not sure based on what you've said. This setting tells MC what to do when it can't see a file any more. By default, MC removes any file it can't see. But that does not seem to describe your issue. You say all files are unplayable. Do their icons have red (-) signs on them? The red minus sign indicates that MC can't find the file where it thinks it should be.
A screen shot or two might help here.
Good luck.
Brian.