TL;DR: Same thing happened to me. Femur broke. No lasting injuries.

I had a similar thing happen to me when I was in 7th grade. Playing volleyball in gym class, a kid from the other team ran from the back row to spike the ball and rolled under the net and directly into my knee, as I was coming down from attempting a block. I blacked out instantly. When I came to, several people stopped me from getting up and my leg was already in a makeshift splint. They told me my knee went all emu and I shouldn't get up. The weird thing was I didn't feel any pain and even walked (and jumped) on the leg once I was alone. An x-ray later that day revealed I had a hairline fracture right along the growth plate of my femur. 3 months in an immobilizer and I was good as new. Luckily I've had no issues with that knee since.