This happened to a merino wool v-neck sweater of mine too. I went looking around online for answers, and someone (somewhere, can't recall where I read it) suggested soaking the sweater in water with a bunch of hair conditioner added. Don't rinse it out totally, and then stretch the sweater back into place (ie. on an ironing board). It worked for me. I purposely bought some really inexpensive hair conditioner at the local grocery store to try this 'trick'. I bought Alberto V05 Moisture Milks Strawberries & Cream moisturizing conditioner for something like 98 cents I think it was.
I figured it was worth a try, and I'm happy to report that it worked on a regular v-neck pullover that had shrunken to the point where it was almost a cropped sweater. I filled the kitchen sink with cold water, added about a half cup of the cheap conditioner - let it soak. Swished it around a bit. Drained sink, and only very lightly rinsed it out using the spray on the faucet. Transferred sweater onto a bath towel on the kithen counter, rolled it up. Took the towel with the sweater rolled in it downstairs to the laundry - unrolled, pulled it back into shape and let it air-dry on the towel on the ironing board. My sweater went back to 'normal'. (due to my pulling it back into shape while it was wet/damp and softened up with hair conditioner).
I can't promise it will work for a cardigan, but for $1 and minimal effort, it's worth trying.