What Causes Sudden Weakness In The Left Knee?

My left knee buckles sometimes when I go down the steps or do some walking. There is no warning it just gives out. I have fallen 2 times down the steps but thankful, I was able to grab the railing and had mininimum bruising to my toes and ankle. I could be walking fine and all of sudden my knee gives out and I am on the ground. This scares me since I could fall in a store parking lot or in the park. What is causing this?