If you wash a sweater that is 100 percent wool, the warm water and agitation can cause it to felt and shrink. Felting is a result of the wool fibers rubbing together, which can erase the visibility of the knit or crochet stitches. While a sweater that has felted will never look like it did before due to the alteration of the fibers, you may be able to stretch it back out with a process called blocking.
Wash the sweater by hand in a sink full of cold water. Use a capful of silicone-free hair conditioner instead of soap, and stretch the sweater gently in the water without agitating it.
Roll the sweater like a jelly roll in a large absorbent towel, gently pressing out any excess water.
Cover a soft surface with another large towel. A bed or sofa works well. Lay the sweater on the towel
Stretch the sweater back into shape and pin it all the way around the edges, inserting a pin every 1 to 2 inches.
Allow the sweater to air-dry completely. Depending on the thickness of the wool and the climate, this can take up to two days.