Swelling and pain in Knee. Are coming.

I have swelling and pain in my left knee since 3 days. 3 Days ago, when I was standing suddenly my leg bones twist and I fell down. This cause extreme pain at that time and I couldn't stand , sit , bend &straighten my legs. Next day pain became less but their was extreme swelling on knee and around knee cap due to this I couldn't bend and straighten my leg. Yesterday when I sat on floor and bounded my legs then, when I stood up I was unable to walk or straighten my leg and their was extreme pain. I had tried medicines for swelling and pain but they didn't worked. Now I am unable to bend and straighten my leg with little pain and extreme swelling. So tell me what's problem and how it can be cured.......please tell me immediately my board exams are coming..