You should probably try a more competent repair shop. If the brake pedal
went to the floor then there has to be a problem. It may be an intermittent
one, but it is a problem. When your brakes fail due to overheating it is
only after some very hard use of the brakes. The friction of lots of hard
stops can cause heat to build up in the fluid and bring it above its boiling
point. When that happens, the brakes don't just go to the floor, the bake
pedal gets mushy and the brakes do not work very well, but nothing like you
described. There are several things that could cause an intermittent loss of
brake pedal. It depends on the year make and model as to what those things
might be. A competent Technician should be able to figure out what could
cause this to happen on your car and should be able to check those things to
determine what happened. Please do not go driving around in a car that may
suddenly have no brakes without warning. Consider the incident as a warning
that there is a problem and get it fixed.
I would suspect the most likely cause would be a seal in the master
cylinder, but some tests would have to be done to check it out.