[HoopyFrood]

Would it have mattered? Is magic affected by things getting in the way? In The Goblet of Fire, Harry managed to summon his broomstick, even though it was tucked away in the castle (maybe in the Gryffindor tower or somewhere). Perhaps if the accio spell had worked, the sword would have floated up then he could have smashed the ice to get it...or, if the spell was powerful enough, it could have smashed through the ice itself.