My knee cap went out of place, cracked, and i fell off the treadmill. it hurts when i walk, like it still needs to crack? what's wrong?

Possible patella sub: It sounds like you may have had a patella subluxation. Most subl/ disloc occur laterally (moves outward), & if this is the case with you, then i would recommend rehab/ physical therapy focusing on medial (inside) musculature to try and counter ligament laxity. There is also a brace that you may find helpful. Before returning to exercises have a x-ray to check for bone or cartilage damage.

Patellar Dislocation: While it may be nothing serious, you may have dislocated your knee cap (patella). If this did occur it is possible that the patella is still dislocated but this is usually obvious. Even so, there is a 10-20% chance that you knocked a piece of bone or cartilage loose and is floating around in your knee. You should be evaluated so that the injury can be diagnosed and treatment can be initiated.