I Was Sitting On The Floor And I Went To

I was sitting on the floor and I went to stand up but as I put pressure on my right leg, my right knee popped and I was in so much pain, I rolled onto my back and out of adrenaline I hit my knee with my hand. After hitting my knee I felt a crack and then the pain had faded but as I stood back up the pain came back and now my knee is swollen. What happened to my knee ?