Stretched-Out and Shapeless V-Neck
On acrylic and rayon sweaters: Droopy necklines in these fabrics are a difficult proposition. These fibers stretch easily and usually can't be returned to fighting form, especially if they have been tumble dried at high temperature.
On wool and cotton sweaters: Try blocking the garment (or ask a dry cleaner to do it): Dampen the garment, lay it out on a flat surface in the shape you want, then press with a towel to remove moisture. (You can also stretch a too-small sweater this way.)
In the future: To prevent excess shrinkage or growth, Joseph Hallak Jr., owner of Hallak Cleaners and president of the National Cleaners Association, suggests washing sweaters in a mesh lingerie bag, which will minimize stress and friction on the fabric over time. And never overdry knits.
Odds of revival: In your favor for natural fabrics, but don't bet on synthetics.