What Is The Treatment For Severe Knee Pain?

i am having terrible knee pain since last 4 days .. yesterday it was too much that could not even walk. Now i can not sit with folded legs as well - it hurts my knees ..? while standing i feel as if my knees are not able to take my body weight ..though i am only 51 kgs with a height 5feet 4 inches. Can you tell what can be the reason