What does it mean when your knee hurts?

Ok so these 2 weeks the back of my knee has been hurting really bad and this morning I woke up and it hurt bad and like when I straighten my knee I feel pike my knee cape will like fall off and my mom said that you might to stretch it and I tried that but it feels like my knee cape will fall off and ya so does anyone might know what will help it ow ya and I can't put pressure on it or walk thanks. : )