I would send it back, either to Amazon for replacement or Canon for repair, should still be on warranty? The lens should not do that, unless you dropped it and jammed the gears. If it jumped off the track once by itself there is a good chance it will jump again, even if you fix it. If it's not on warranty then you could try to force it back in or take it apart. Paid repair at Canon could be little expensive, so I would call first for estimate, maybe buying new lens wouldn't cost that much, after all this is a kit lens, almost given away with most entry level DSLR .