Upon reopening a project last opened several months ago, I receive a 'Missing Plug-Ins' error message and a list of all the VST Synths in the project which Sonar can't find (nine instances total).  Of note is the fact that I had changed the 'name' field of each of the missing plug-ins in the Synth Rack. For example, Play 1 was renamed Winds 1, Play 2 was named Brass 2, Perc 3, VLN-I, VLN_II, etc., according to which instruments were loaded in each instance of Play.  Although Sonar allows me to do this, I've run into issues with this before, and I now suspect it was a bad idea.  Anyway, I think the supposed missing plug-ins are not really missing, Sonar just somehow "forgot" that Winds 1 is Play 1, VLN -II is Play 5, etc.  But now, I don't know how to fix it, other than painstakingly recreating each instance and then figuring out which exact instruments were loaded, etc. Over 80 tracks, so this is not an appealing option.  The Synth Rack shows all nine instances, but the 'power' buttons are grayed out, and I can't access properties, etc.
Any ideas how to "relink" the synths to the proper VSTs? I did a complete VST re-scan of all plugins, including "failed" plugins to no avail.  Thanks!

The plugins have to be on your system somewhere. I would search for them and then rename them what they're "supposed" to be named. Then try another scan. You can just use the search field on your Windows start menu. Search for both the correct names as well as the new names. When you find them, right click and look under properties, it will tell you where they are hiding. I apologize if this is so basic as to be insulting, or if you've already tried it. but it's the first thing I would try.

Thanks - no apology needed. The plug-ins, in this case either East West Play, or Garriton’s Aria player, are not lost. They are both properly installed and working on my system. My theory is that Sonar is looking for a synth .dll named “Brass” or “Winds”, etc., but can’t find them because they don’t exist. The original .dll files are still properly named and installed in the correct locations. I can still load a new instance of Play or Aria without any problem. (The purpose of my renaming the Synth instance was to remind me which instruments are loaded in each instance. )

Since Sonar allows you to rename the Synth, it must have a mechanism to “link” to the correct .dll. This has worked fine for me the majority of the time. I do recall one or two isolated times when something similar happened, and I chose to reload the instruments from scratch. This is a much larger project, and will do that only as a last resort.

So again, it seems that Sonar has “lost” the link between the name in the Synth Rack and the actual .dll file it’s supposed to point to. I have no idea how to “re-point” it. Or, maybe worse, this could have nothing whatsoever with the renaming, and this is purely coincidental to the actual problem. However, the project in question has an instance of Ivory in it, and it opens fine. (I did not rename Ivory since I knew that it was a piano). Hope this makes sense. Any other ideas?

Sorry, I misunderstood your question (not an infrequent occurrence. Oh well, once in a while I say something helpful...