That's unnerving! Could you explain a bit more about what happened?
I believe the braking system is "brake by wire", i.e. no physical connection to the brakes from the pedal, which allows the ECU to use regeneration as much as possible and then apply physical brakes if required. Sounds like maybe it only used regeneration...??
More info: http://gm-volt.com/forum/archive/index.php/t-50697.html
Seems that if this happens, put your foot *hard* to the floor to engage the back-up direct link to the master cylinder and also put the parking brake on all the way (keep button pulled back for several seconds)...
The computer showed a fault stating something like 'service required'. I just did a reboot and all was well again, but it was scary as there was no pneumatic help so you had to stamp on the brakes to stop.