I live in Ohio, not quite as cold as Michigan, but I have a pond about 18 inches deep and the frogs seem to winter OK.Spring and I have a good crop of tadpoles and the larger frogs are still there. Throughout the winter I keep a hole in the ice. There is an frog FAQ @:
Http://www.learner.org/jnorth/search/FrogNotes3.html

 If any experts out there think this is wrong please correct me, but I've since learned that pond frogs (not sure exactly what species I have in my tiny pond) typically stay in the water thru the winter and actually can come close to a frozen state, then "thaw out" in spring. What little oxygen they need they absorb thru their skin. They do NOT burrow into the mud at the bottom as turtles do--if they did they would suffocate. Therefore it probably makes sense to leave him in the pond and hope he makes it thru winter, and finds a mate in spring!

 Most of the times the frogs will go into hibernation and you won't even see them anyways until spring. I think that they will winter ok and you will be seeing more of them real soon.