My knee hurts bad. it's swelling & i can't stretch it out or walk on it. && occasionally it will lock in place .what's wrong with it?

Torn Meniscus?: That sounds like a meniscus tear, cartilage in your knee. The tore fragment is getting pinched causing pain and locking. See your doctor and will get MRI. This can be repaired but if left can cause more damage. The repair is usually done through a small scope so recovery is quick. Good luck. there can be other causes but all need attention.