My right knee grinds. i can't put major downward pressure on it (going down a ladder) as it quickly gives out followed by numbness/pain. why is this?

See a doctor: You don't give enough info for a diagnosis. Duration and known recent injury would matter. Your description sounds concerning for meniscus tear. Other ligament injury or fracture also possible. These are permanent injuries which will not resolve on their own. I would stay off it, buy an OTC brace for support, take OTC pain reliever, use ice liberally and see an Orthopedic surgeon.

Knee: Sounds like you may have an issue with a meniscus, which is the cartilage between your femur (thigh bone) and the lower leg bone called the tibia. It acts as a shock absorber or cushion between the two bones. I would see your doctor and consider getting an MRI to view it closer pending an exam of your knee.