If an opponent casts a creature spell with flash while Weeping Angel is attacking or blocking, it is removed from combat because it stops being a creature. However, any creatures that it was blocking are still considered blocked.

Damage that is prevented never occurs, so any triggered abilities that trigger due to combat damage that would have been dealt by Weeping Angel won't trigger.

As the first triggered ability of Weeping Angel resolves, it continues to be an artifact but will stop being both an Alien and an Angel until it becomes a creature again. It's just a statue, how dangerous could it be?