'힝' is a kind of sound we make when we express stressful, annoying, or depressing emotion. It is used by women usually. ex) 어제 나 시험 망쳤어~ 힝~ ㅠㅠ 힝~ 보고싶어!!

It's a cute way of expressing your disappointment, depression. A : 나 너랑 안 놀아! B : 힝 ㅠㅠ A : 이거 좀 도와주면 안 돼? B : 어, 안 돼. (joking) A : 힝 ㅠㅠ