Electrical issues

The battery light was telling you the generator was not recharging it and it was going dead. The loss of power was the result. Much of the battery's charge is stored deep within the plates, and it migrated out while the engine was off. That's why you were able to start the engine later.

If the battery light was off later you have an intermittent problem that was not acting up at that time. That is commonly caused by worn brushes in the generator. They can be replaced as part of the voltage regulator bolted to the back of the generator, but most people just replace the whole unit.