I must have hit a button somewhere along the way.  When I press record (to record some new info onto a track) I cannot hear the audio I already have recorded... it goes silent.  

Are you recording it onto the same track as the pre-recorded audio?
 
If so, make sure "Sound on Sound" is selected from the record button.

That worked thanks... not sure what I hit to disengage it.

OK... now I can't hear my guitar on playback or when I record (as I'm recording)!!!!

OK... now I can't hear my guitar on playback or when I record (as I'm recording)!!!!

If the above is not clear... I can hear my guitar so long as I don't record anything or press play.  Once I do either, my guitar playing goes silent!!

OK... more friggin' around and I can hear my guitar when I press record... and I can hear it when I don't press play (to hear what already is recorded).  But I cannot hear my guitar if I simply want to play over a track. I have to press record for my guitar to be audible.  What the frig happened?  
 

You can just arm the track and hit play to practice over the backing track.

Is Input Echo 'On' for your guitar track?
 
You need that turned on for the guitar track, in order to hear your guitar during playback and/or record.
 
Bob Bone