Hearing no sound when I record midi through softsynth

I've recently come back to Sonar after a couple of years, and I've upgraded to the new Platinum version.
 
I started a project using midi tracks and the Cakewalk TTS-1 softsynth. I recorded about 10 minutes of music. All successful.
I then proceeded to explore the features of the program and to learn a few shortcut keys etc - nothing drastic I thought.
I then wanted to record more music in the same file. The existing tracks play back fine. But now when I record I don't hear the instrument that I'm recording, but I when I stop, all the midi data is there and it plays back fine even though I couldn't hear it when recording. As I'm recording I do hear any other tracks that have data at that point. If I open a blank file, load the softsynth and create midi tracks from there, it all works as expected.
Does anyone know what have I done? - I know from past experience one false key press on the computer keyboard can cause mayhem :( I don't really want to start a new project as it took quite a while to set up the midi tracks with the correct channels and sounds etc. But I will if it's the only solution...
Any help gratefully received!
David

thanks for your reply.
 
When I go to record I click the track to get the correct sound, then click the arm button, the icon next to it on the right with the dot and ripples extending from it also lights up, then I press R on the keyboard and off we go. This is exactly the same procedure that I used on the 502 takes before.
One thing that I have just noticed when I went back to check how many takes I had done is that the new takes have started numbering again from 1... Could this be a clue as to what I did wrong?

Double check the MIDI routing on that quick. I have noticed issues where returning to previously recorded tracks with takes already recorded had some routing gremlins going on. I forget what exactly I was doing with it at the time, but it ended up being repeatable. Quick checks to try:

1. Enable "All" (versus "Custom") at the top of the Track View (TV).
2. Check the MIDI channel, inputs and outputs on both the MIDI track/synth.
3. If you arm and Input Echo the track are you hearing anything before you record?
4. Reset the MIDI and Audio and/or toggle the audio engine off/on (both in the Control Bar/Transport Module).

IIRC, it boiled down to MIDI routings "broke" but forget the exact details with it. Steps 1-3 above tend to be good practice prior to recording any synth track.

That actually sounds different than what I initially interpreted. Were you recording to an aux track, perchance? I forget the exact behavior now, but when aux track recording was implemented there is muting going on to prevent a feedback loop.