Ruckman65
JohnD - I tried your suggestion of taking the HW keyboard out of the equation and using a virtual keyboard but the problem persists.
 
Mike, there is activity in the channel meters for both audio and midi. The signal routed through my Sound Canvas module triggers midi and I can hear all midi sounds. The problem is with the VSTi's. Midi activity is shown and the channel meters register the signal but there is no sound! 

This sounds more & more like a routing problem, especially if you see audio  being triggered.
 
Are the vsti tracks routed to busses? could you describe your bus structure

So a reset of the keyboard does not solve the problem. Still could be a stuck controller switch on the keyboard that is sending a message when it is connected. You could download MidiOx and find out exactly what is being sent by the keyboard, or look in the event list to see if there are events there that do not belong. Or the grandchildren playing with the keyboard could be a coincidence/red herring. Or something has been changed in your setup that persists across a restart of Sonar. Are your VST's capable of retaining settings after they are not in use? Open the synth properties and see what they are showing. Are you opening new projects using a template that might have been altered? Try opening without a template.

Bristol_Jonesey - It certainly does sound like a routing problem, I agree, but, on the surface, everything looks the same as it did prior to the problem. If I look at the routing of older projects, they look the same as those in a newly created project. The difference is, the VSTi's work in the older projects but no the new ones.The VSTi's are not routed to busses.
Slartabartfast - I have been using MidiOx to see what is happening and all appears to be OK. The events it reports match up to what ever it is I am inputting on the keyboard. The issue applies to every VSTi that I have so it appears to be a global issue rather than VSTi specific. I do not use templates to start new projects.  

Thank you all for your help and suggestions. I bit the bullet, reformatted my hard drive and started from scratch. Pleased to report that, after re-installing Sonar and all VSTi's, everything is back to normal (so I may never know what the actual issue was).

Glad you are up and running again 
To bad we did not get to the bottom of it without a full system installation (could have been useful knowledge).
Best of luck.