What problem did Prometheus and Epimetheus face when they came upon man?

Humans were cold and couldn't protect themselves.

Explanation:

Once Epimetheus had finished quickly making all of his creations (animals) he realized he had given away all of Zeus's gifts, so that when Prometheus had finally finished carefully sculpting his humans they had nothing, without coats of fur they were cold, and without claws or fangs they couldn't protect themselves from Epimetheus's creations. This is when Prometheus went to ask for fire as a gift for the humans.