I would get an appointment to see an Orthopaedic Surgeon, could be your meniscus or a ligament. I've just had a reconstruction on my ACL and PLC, if you cant bend your knee then something is stopping you. I have a friend who was just standing at work and suddenly his leg went and he had tore his ACL. Sometimes these injuries can be that simple, he walked on it for a few days before he knew something was wrong. You could have quite easily twisted your knee whilst stepping down. Good Luck Jo