Final answer:
The bad omen in the ballad 'Sir Patrick Spens' that frightened an old sailor was the sight of a new moon with a white face, which was considered a predictor of ill fortune or a storm at sea.
Step-by-step explanation:
In the ballad Sir Patrick Spens, the omen of bad luck that one of Sir Patrick's sailors saw, which made the old sailor afraid to sail, was the new moon late rising and looking with a white face. This was taken as a sign of an impending storm or disaster at sea, as folklore and maritime traditions often involve the interpretation of natural phenomena as omens.