Excruciating knee pain?

about 6 years ago (i was 14yo) i tripped over a piano dolly while playing basketball in a hall, my knee was the first part of me to hit the wood floor, it was sore but the pain subsided, but now i am 20 and when i kneel on the front of my knee i get excruciating pain, i don't know whether it is a chip or a pinched nerve or what, i wondered if any of you have had a similar experience ? thanks heaps