AC power leaving the circuit breaker, but no AC power to AC power supply line in bathroom (and this bathroom is head/lead in daisy chain)
First, I want to thank those of you who responded with the excellent advice.  And after checking the continuity as suggested, there was continuity!  

However, there was no continuity of the neurons in my brain until when I realized that the two upstairs bathroom receptacles may be upstream on the same daisy chain 20A circuit for the two downstairs bathrooms. They were- I am an idiot. 

But in my neophyte defense, the electrical panel had two separate circuits dedicated to these two upstairs bathrooms labeled "Upstairs bath #1" and "Upstairs bath #2".  I assumed they were for the receptacles and lighting.  Clearly the upstairs bathroom lights and receptacles don't share the same circuit as most don't want the lights to go out when the GFCI trips the receptacles.   

To keep two separate GFCI branches on the circuit (one branch for the two upstairs bathrooms, and one branch for the two downstairs bathrooms), the hot line and neutral wire from the electrical panel goes to upstairs bath #1 GFCI receptacle.  The lines are inserted into the proper respective line holes on the back of the receptacle (they are clamp and screw, not pure backstab).  However, there is also another neutral/hot pair (from/to the downtairs guest bath GFCI receptacle) that is connected into the respective line holes on the back of the upstairs bath #1 GFCI receptacle. So I have two neutrals and two blacks into the line side screw/clamp holes of the upstairs bath#1 GFCI receptacle. And on the same upstairs bath #1 receptacle is the load wire connection to provide GFCI protection to the downstream bathroom #2 receptacle.

This was maddening to figure out, but I understand it. 

I had to replace the upstairs Bath #1 (the "workhorse" supply power splitting, load carrying) GFCI receptacle because the hot/black line insert/hole carrying power to the downstairs guest bathroom was shot.  Why was it shot? Likely because of my wife taxing, for the past three years, the 20AMP circuit and our master bath 15 AMP outlet (on the same circuit) with: (1) her overpriced salon hair dryer (which could dry a Wookie in seconds flat and sounds like an F14 engine), (2) and a "hair flattener" curling iron device, and (3) a clothes iron - all on and running at the same time!  She had a small mini three outlet strip plugged into the 15Amp outlet to run all of these items at the same time.  The mini three outlet strip is now in the trash. I had no idea she throwing that many amps (at the same time) at the circuit and the outlet! And I installed a 20AMP receptacle in our master bathroom and explained why we don't want to turn all those high amp items on at the same time on the same circuit.

Sorry for the long post, and thanks again for the excellent advice.  

thanks.