Looking for information on where to start with brake problems.  Brakes were almost locked one day and now the pedal goes to the floor with no brakes at all.  Master cylinder has fluid.  Can anyone give me a place to start diagnosis?  Start by bleeding procedure or look at the individual wheel cylinders?  Just wondering where to start with the brakes going from seized to nothing at all.  Thank you.

I’d start with the rear wheel cylinders. Maybe take a peek looking for fluid running down or the stain from it. That might determine which side easier. 
 

And if you’re in the rust belt like me you want to check those rear brake lines, the passenger side usually the culprit. 
 

And the M/C stays filled ? Both reservoirs? 

Thank you.  Yes both reservoirs on the MC stay filled.  I will take your advice and check on the rear wheel cylinders.

I know this is old but I would just like to say that it sounds like a seal or line has ruptured, probably from the force of attempting to un-seize the brakes.