Hi I Fell Off Of A Basket On Halloween Night,

Hi I fell off of a basket on Halloween night, and when I fell on to my left knee on the outside part I heard a poppin sound. Now to date I am only able to apply little weight to my left leg and sometimes I feel my knee buckling to the inside. I am not able to bend my knee all the way at a 90 degree angle, I have had to X-rays and they both have came up negative...what is wrong with my knee?