Can't straighten the knee since falling on the floor what could be wrong?

Knee injury: I assume pain limits your knee movements rather than knee dislocation. If you note no deformity, elevate your leg and apply ice wrapped in a towel for 20 minutes several times a day. If you can take them, anti-inflammatory medications like ibuprofen or naproxen reduce pain and inflammation. If not gradually improved or in severe pain or swelling, go to the emergency room for prompt evaluation.