Ok, deep breath. This is bound to happen if knitwear rubs up against something sharp or textured, or if a purse or jewelry piece was acting extra-malicious. DO NOT CUT THE YARN!  There is a little tool you can get on amazon to help repair snags/pulls. It's available right here and is basically a pointy metal stick with a bunch of texture on one end. You push the point through the part of the sweater that has the snag/pull and then pull it through the knit to the inside of the sweater. The textured part catches the extra yarn and travels with it to the inside. (This snag repair tool is also available at like all sewing stores!!) We get extra fancy when we repair our own sweaters and actually angle the needle through the garment. So the excess yarn is pulled through the entire sweater rather than just to the back. Please be careful if you're going to try and be this extra. Not only because you can poke yourself with this needle very easily, but because you don't want to pull the yarn too tight!