Sonar X3 - Just one song - Kontakt and Elastik player midi tracks have gone silent

Hi all
 
I'm having a problem where I loaded up an old song for the first time in a few months, and though the tracks and midi data for Kontakt and Elastik Player are still there, there is no audible sound. 
 
I've obviously checked to see if the tracks were muted but they aren't. The music was created on all the equipment I have now, so I didn't change anything. A strange thing is that I can create fresh midi tracks and they play fine. It's like there is some sort of broken link. 
 
Does anyone have ideas what might be wrong and how I can put it right? 
 
Thanks in advance
 
Andy