From your description, I think your frog flipped over by accident. However, if your frog flips on his back often, then there is need for concern. I observed this strange behavior several times when I worked at the zoo as an amphibian keeper. Nearly every day, I saw several frogs on their backs struggling to right themselves. Other than that, the frogs were in good health. I brought this up with the other keepers and thus was our conclusion:

1) The frogs sensed some fear, whether real or perceived (Unken reflex)
2) There is a neurological problem

I wouldn't worry right now.