Lost a sneaker

This again is another situation that could be handled differently and still be acceptable either way.
Based on the scenario in the OP, for anything below a JV or V game (because that's all I've done), I would most likely stop the game and allow them to put it back on.
For these lower level games, I agree with Rich. Common sense and fair play is what it's about.
For higher levels, it really all depends on the game at the time.