I run Melodyne singletrack inside SONAR platinum. It has been working normally for the last week.  
   
Today, the polyphonic display dropped into mono, the gui grayed out, and it presented me with the licenser screen.  

I tried uninstalling/reinstalling, and it work for two minutes before dropping out again.  
   
The only software change has been the monthly SONAR update 48 hours ago. Any suggestions what
I might try next?

When you say Melodyne singletrack, I assume you purchased that separately direct from Melodyne.
If yes,  I have that sort of set up (although I did think mine was called editor??).
If you accidentally install the Sonar version I do believe all sorts of weirdness can happen - perhpas this happened on the update - I remember another thread about this issue - perhpas a search might find it - cheers
Nigel