It sounds like you were out of fuel, right?
If this was the case the backfire was caused by a weak fuel mixture, the ratio of air was to high. This causes the fuel mixture to become less stable and will ignite as soon as it enters the cylinder, without or before the spark!
A constant weak mixture can cause over heating, backfires out of the exhaust, though the air intakes which can cause a fire and last but not least engine damage.
A one off is unlikley to cause any damage so don't worry, bet it was loud though.
