Can not straighten knee or put weight on knee?

Inability to straighten the knee or bear weight on it can indicate a serious issue, such as a ligament injury, meniscus tear, or patellar dislocation. These conditions may result from trauma, overuse, or degeneration. It's important to seek medical attention for a proper diagnosis and treatment, which may include rest, ice, compression, elevation (RICE), physical therapy, or possibly surgery, depending on the severity of the injury. Prompt evaluation can help prevent further complications and facilitate recovery.