Weird problem - mixing half way - many tracks gone silent

I was mixing my forth or fifth track and moving between the time line and suddenly most of the tracks gone silent (drums, vocals, bass, etc..) and only a few was left as normal.
 
Thinking it's just a software glitch, I save the project, restart Sonar / reboot / etc.. but it stays the same now.
 
Turning off all the effect (pressing 'e') restore the volume of all tracks.
 
What is the best way to rectify the issue ? I was having fun mixing and boom.. 

Thanks scook. I shall check it out later (going for a rehearsal now).. just worrying working on a track envelop can effect other plug-ins.. (that's a likely issue, since turning off all effects restore all the tracks volume).