I fell at work, hard on my left knee. i have sharp pain when kneeling or even when i touch my knee cap. what happened?

??: Maybe you broke your patella or possibly damaged the cartilage on the under surface of the knee cap seeing a doctor seems indicated.

Need X-ray : If you haven't done so already, you need to be examined and have an x-ray. It is possible that you have fractured/broken your knee cap which sometimes requires surgery or at least a brace or cast. If not, then most likely you have sustained a contusion or bone bruise which should improve with time. Other possibilities exist as well & are best determined by an orthopedic evaluation.