No sound on playback

I have a problem with two versions of the same project that I can’t solve. I have recorded a number of instrument tracks and put volume automation on all but two of these tracks. While I’m working on the project all is working well, sound is coming out fine, automation works fine. I turn off the computer or put it to sleep. On rebooting the PC there is no sound at all from any of the automated tracks. The two tracks with no automation produce sound but not the automated tracks. I can see the MIDI signals are getting through to the VSTs and the automations are working but no sound.

This has now happened twice with two different versions of the piece. All other projects appear to be working OK, just not this one. Anyone any idea where I might be going wrong.