Why did car lose all power when engine was still running?

All power or most power? When an engine's alternator (which is a generator for everything electrical on a vehicle) goes bad, the vehicle tends to run until the power in the battery runs out. During this process, all the electrical components (radio, lights, etc) start dimming due to the engines electrical system giving priority to the ignition system (spark plugs, ICM or Distributor, etc). (Side Note: the battery is only used to crank the engine, not to run it.)

Now if it loses all power, with the engine running, then I have no idea. There must be a major short and or quite a bit of fuses burned out.