3 weeks ago i sprained my knee on concrete sidewalk. i still can't fully bend my knee without having pain. it is painful all around top part of knee/ ?

Get examined: to rule out a fracture and get an MRI to rule out internal derangement of the knee joint. I suggest an orthopedic evaluation. Pain and severe limitation of range of motion are suggests of fracture or tendon/cartilage damage- you MUST be examined.

Be seen: It may not have been a simple "sprain", you may haves sustained a more serious injury to the meniscus, ligament or tendons of the knee. You would be well served to be seen to make sure it is not a more serious injury.