There isn't much you can do to fix a shrunk sweater. Once the fibers have seized like that, there is little aside from gently stretching them that can get them to release once again. However, by soaking it in a lukewarm bath of water and gentle hair conditioner, taking it out of the bath and gently shaking it until it stops sopping water everywhere, and then laying it out on a towel, you can begin the process of stretching it back to almost normal. Lay another towel on top of it, and gently blot it to get some of the excess moisture out, however, it should still be fairly wet. Now using the towel, begin stretching the sweater lengthwise and width wise until it is slightly larger than it's normal size, place the towel over the top of it, and let it dry in a sunny place. Wash it to get the conditioner out, and dry as the tag suggests.