I record an audio track.
I can hear the track as it records.
The waveform is present and it is at an appropriate gain level.
But it does not play.  No audio at all.
All other tracks are fine.

Lots of things to check, start with track effects bin and then Pro Channel - one of the effects may mute the signal.

Check ALL of your routing, from track to bus to main outs
 
Are your meters showing a reading when the track is playing?

Maybe clip is muted? Key binding to toggle muted clips is k

To save time, I would create another track, copy/paste the wave file and work from there. 

It's situations like this that would make a routing map handy....

OK. one of the things that make the impossible, possible. Is the LETTER O..... if you hit the letter O by accident, it looks like nothing happen and away you go but what it does do is toggle the manner in which the CONSOLE behaves.  It puts the console into relative mode or something like that which only effects what the console setting may have been before you hit the letter O.
 
So, if the fader was completely off when you hit the letter O and now you go to use the fader moving it up or down has no effect since it was complete OFF in its native mode.  Same thing for panning.  This letter O has driven me mad too many times to count but every time, something I thought was impossible to happen got explained once I realized I hit the letter O by mistake.  To put it back in the regular mode, all one has to do is hit the letter O again and you will see the real settings for the track.
 
When the letter O is active you see some squiggle makes to the left and the right of the top of the fader.  And I am sure that this mode serves a purpose but I wish the bakers made it harder to turn on other the hitting the letter O.
 
My OOOOOO My what a PITA this is.
 
Jim

Thanks for the helpful suggestions.  I could find nothing.  No muted clips, no automation nodes, not a thing in the event list.  I typed the letter O.  (The O thing is scary)
 
I started a new project and dragged all the tracks over to the new project and....problem solved.
 
Go figure.

I had no idea. "O" is for Offset mode. You can tell when you're in offset mode when all values have a "+" after them.
 
Offset Mode
 
Type or copy/paste: http://www.cakewalk.com/D.umentation?product=SONAR X3&language=3&help=Automation.13.html

I thought the letter O was no longer associated with Offset mode?
 
The only way to invoke it (AFAIK) is by using the dedicated button in the Mix module of the command Bar (unless you have specifically mapped O to Offset)