Hi Norrie,
What do you mean, exactly, when you say that you have lost your MIDI tracks? Is it the MIDI clips on a track that appear to have been deleted? Or have the actual tracks gone missing? Could it be possible that you have accidentally "hidden" a track?
By the way, unlike Audio clips, that are saved in an external folder somewhere, MIDI information is stored "inside" your Sonar CWP project file. So there is no way of rescuing that, unless, as the other guys have said, you have had autosave set to on.
I wish I could help you Norrie, because I know you are still picking this up, but it may not be possibe...