The fight

Danny is proclaimed "dead" by his parents after passing out from exhaustion after a long drawn out fight with a ghost. Do too his ghost half being unstable it causes his human half to appear deceased.

Danny slipped into a comatose state and his ghost half glitched. Afraid that Danny may turn into a ghost like the ones they had been fighting they made a special coffin for him and they put deep in the ocean. Anyone who asked was told that they had held a private funeral and there was a fake grave with Danny's headstone on it as well.

Someone finds the coffin, someone bat-like.