It's extremely common for an engine to run backwards. You won't have done any damage to it, and it's obvious when it happens as the engine is pushing in the wrong direction (obviously!)

Sometimes you can make it run in the right direction by flipping open the throttle quickly, in which case it 'backfires' and starts running in the right direction.

Don't worry about it. It's one of those things that happens with a model engine.

-David C.