Knee Hurt For Month. Painful To Use After It Has

Knee hurt for month. Painful to use after it has been bent but then gradually gets better as I walk. However, today it made a loud popping noise - with intense burning sensation. (Felt like when I tore ligaments in ankle). No bruising. No swelling. Yet I now cannot put weight on my leg at all. Also painful when I bend knee. Not painful if I lie down and keep it still. I am 50 and not sporty. No injury/incident that triggered it.