What to do about a hyperextended knee?

i seriously hyper-extended my knee i was playing soccer and i fell and when i tried to get back up really fast idk what happened but my right knee flew backwards really fast and hard basically with all my weight because ii was trying to stand up. and that was yesterday and it still hurts like crazy! i can barley stand on it and i can bend it a little but bending it alot hurts like crazy. the outside of the knee hurts the most. when i stand my knees normally go backwards but that knee is just straight and it feels weak. ive tried RICE but hasnt worked yet. what should i do???