Did he say why? Sometimes maybe they just wernt really excited when they started, or alot of other things could make him just not hard at that moment, doesnt mean oh he thought you didnt look good or something, but you shoulda asked, are you not turned on, or just not in the mood, whats up? Maybe all it coulda taken was a sexy pose or position he could see you in to get his blood flowin again, you need something to get you excited! Don't get to crazy over it, just talk to him, or forget it, and if everything goes back to normal then cool, and if the situation happens again, then ask him on the spot. Hope I could help in someway.
Best of luck to you, take care. :)