Most likely it isnt that the heart stops or starts in response to freezing. The heartbeat signal probably runs 24/7 unless inhibited by freezing. So as soon as it thaws, the signaling bit resumes its pulsing and the rest of the heart begins to beats as it also thaws. The order in which organs thaw would be regulated by sugar levels. Remember too that at this scale frogs can absorb much of their needed O2 through their skin. A steady heartbeat isn't as essential as it is with us.