What kind of material is it made from? Many of the synthetic fabrics have different washing qualities to wool and I suspect that once it's gone out of shape there is little you can do to make it revert. After much experimenting with other fabrics I now try only to buy 100% wool jumpers, apart from fleeces because those with a large quality of nylon, orlon etc. just don't spring back into shape after washing. You'll probably find that tumble drying won't shrink it either unfortunately.