What Causes A Popping Sensation In The Knee Along With Pain?

Over a year ago I threw my knee to the side it has happened in the past but the last time really damaged my ability to walk or bend my knee. Recently it has been popping out of place and I can usually bend it and make something pop in the knee joint area instantly letting me put pressure and walk just fine. Tonight my knee buckles and I feel like the let isn t straight at the joint. What should I do.