31 degrees will not kill V5 corn or younger. So it must have gotten alot colder than 31 in some spots. I can say with almost certainty that your corn had not reached the V5 stage yet. One of my cornfields was planted the 23 of April and it has yet to reach V5 it is very close a person might be able to find a couple thousand V5's out of 35,000 but not many. It laid in the ground almost 3 weeks before emergence. I have never seen 3 inch tall corn reach V5. All things considered if it really is dead it was colder than you thought lower than 31 for sure.