I just had to reinstall Sonar X3 and now a previous project that I had a Melodyne pitch correct on can't locate the plugin.   But I'm sure I checked all possible plugins to reinstall during the process.  How can I check this?
 

Are you able to add Melodyne as a new region FX? - this will tell you if Melodyne is being picked up by sonar

No I can't click on RegionFX it all, it does nothing.  Melodyne does exist under my list of installed plugins though. 

The VST3 version must be installed. The VST2 version will not load as a Region FX.

how do I reinstall or update or re-scan for it?

Assuming the VST3 version is not installed, in the Windows Control Panel Programs panel (where you uninstall programs) locate the Melodyne entry. Right-click on it and select "change" from the context menu. The Melodyne installer should popup. Select Modify, then on the next screen select the plug-in formats you want to install.