Duncan Gardiner

I found the answer: The project had been opened as a MIDI project not a Normal Cakewalk project. This makes sense now I know but why did it let me create instrument tracks and save them? This was a couple of hours of work. A warning would have been useful.

I have a midi track linked to an instrument track. All works fine. And saved! After restart the instrument track has changed to a blank midi track, so the original midi no longer outputs to anything.