Try something like a back-scratcher, which is thinner than your hand, to reach under the appliance.
Turn it on its back with the 'fingers' facing upward. Then, maneuver the scratcher behind the magnet ...
If the magnet is important and you must retrieve it, but cannot reach, or it is stuck to the fridge frame:
switch off the fridge
sidle it away from the wall and tip it backwards
retrieve the magnet