Hmmm, did you make the item out of cotton yarn? Cotton has NO memory and ribbing will stretch out. All is not lost though. Just toss the lovely into the washing machine but don't toss in the dryer, instead, lay it flat on your bed or floor on a clean towel or sheet and manipulate it into the shape that you want. Don't have to stretch it or anything but just sorta squish together the ribbing and let it dry. It should (hopefully) be back to normal. Now with cotton you will have to do this everysingle time you wash it. It's not like wool which has a wonderful memory.
If you've made it from some sort of synthetic fibre, the only thing I can think of is to throw that in the wash and dry it in the dryer on low heat, just till it's no longer damp. Don't hang sweaters or other items as their own weight will stretch them.
If you've made it from an animal fibre, you would hand wash it and then block it.
That's the only things I can think of to get it back to normal for ya. If you want you could get some elastic thread from the craft store and weave that into the back of the ribbing. Just be careful to get it the same width around or it'll pull in here n there n not look as good.