Sonar 6 Producer problem

Sonar 6 has been very very good to me.  Running it on xp. 
Suddenly, I cannot get the play button (or the spacebar shortcut) to respond whenever I have inserted a softsynth. For instance, I can record and playback all the audio and midi (provided that the midi is not run through a soft synth) I want to, but as soon as a synth has been added, the playback does not happen at all. I go into the synth rack view and turn off the synth(s) and it plays again, of course without the synth sounds.    Anyone understand and have a solution?
Thank you so much!

Thank you for your response - It isn't CPU overhead problem - And I have found that it happens with all synth plugins - even the plugins that came with the program - TTS, Session Drummer, etc - as well as the plugins I added-  Ivory, EZ Drummer.  These have worked with zero issues for years - but now, I insert just one of them (I tried with all, one at a time, and simultaneously) and nothing happens when clicking play (or record for that matter). 
 
Here's a twist: I have been able to make things run normally (at times) by opening the synth rack view and toggling the on/off switch on the synths - When it's off, it works, when on, it doesn't.  But by switching over and over, some little computer demon on the inside gives up, and I can use the product again with the synths.  But it will fall back into not working again after a couple of days!  how do you like that?  crazy.

Have you tried bringing up an old project, one you know worked properly the last time you had it open, to see if it does the same thing? 
 
Do plain audio tracks play OK? (Import one if all your projects are 100% soft synths.)
 
Can you freeze or bounce the synths? If so, does the project play after freezing?
 
Does the project not play at all, or does it appear to play but there's no audio output?
 
What I'm suggesting is determining if it's strictly a MIDI problem, as that will narrow your focus.