I reinstalled Windows 10 and Sonar X3 Producer. Now when I open a X3 Producer project from before I reinstalled I get a message that a certain plugin can't be found. It's been reinstalled (Izotope Alloy 2). I may have put the .dll in a different VST folder from the original one in the song project. But in Preferences | VST Settings I have put a copy in all VST Scan Paths. But no luck.
 
When I double click on the plugin in the track I get a message that the plugin is missing or is not installed properly. I have used the Alloy 2 plugin with FL Studio with no problem. How do I find the path where X3 is looking for the plugin. And any other help that anyone might provide. Thank you. 

Hi. Sorry if this is obvious. But have you tried to do a vst reset and rescan? Include existing plugins and failed plugins before scanning. If you have several plugin folders, choose only the parent folders, not each individual folder, as I think it might cause issues. 
All the best.

Check that missing plugin window very carefully... since both VST2 and VST3 versions of plugins have the same name. I had the same "wtf" when I opened a project in X2 created in X3... of course X2 couldn't find the VST3 plugins since they were introduced in X3.
 
Also, in Preferences->File->VST Settings, be sure the VST3 folder is in the scan path C:\Program Files\Common Files\VST3 is default, and also uncheck both of the VST3 Migration checkboxes (this allows SONAR to "see" both versions installed).
 
Just to be safe, I install both VST2 and VST3 versions of all plugins when available (you may have only re-installed one, and the project is using the other). You may need to re-install those again and select both versions.

Thanks, guys for the replies. I finally uninstalled and reinstalled and X3 Producer found the plugin. Should have done that to begin with.

Glad you figured it out. 
All the best.