Track silent

Hi has any one else seen this
 
Import a project to mix from anther SONAR user.
Works fine.
Unfreeze one of the track to make some changes and when you play it back all the tracks are now silent.
Nothing is muted, the levels are set, the buses are all active the master channel is active the routing is correct, I have not put cotton wool in my ears or turned off anything.
 
If you create a new track copy the data from the old to the new, do nothing else, it plays back without a problem.
 
Yes the PC is a dedicated DAW, my sound card is Focusrite Saffire Pro 26 i/o (Original), I can open another project and it all works fine.
 
Any ideas any one?
 
Thanks

Do you have the concrete limiter on any of the tracks/busses esp master buss? I sometimes get complete silence and that turns out to be the CL applying maximum gain reduction even if the settings shouldn't do that.
 
Restarting Sonar or deleting the CL and reinserting cures it for me.

Did anyone ever figured this out? I just had it happen in Platinum - and it persists when the file is saved. Track, clips not muted, equalizer "tube" glows when set to "pre." Pre- sends work, but not post-sends. I think the fader is broke
:( 

Well, I saved it last night and the master solo button didn't fix it, but:
 
1) recreating a mute automation track
2) dragging the automation track to mute off
and
3) setting the mute button back to automated mute
 
did the trick.
 
Switching back to manual mute (with the mute off!) breaks it again.