In reply to Rob Exile Ward: Swelling and locking suggest you could have damaged your meniscus (the cartilage mentioned above) and it's probably torn off. This can happen both from an accident or just wear and tear. It would be picked up with an MRI and you could do with an Orthopod seeing you, these things can be fixed, sometimes with very, very small surgery if you choose to do it this way. Physio (as mentioned, private, I'd advise against NHS) will help prevent this happening in the future and speed up recovery but it's not a miracle solution. I'd advise laying off skiing as well, like some have mentioned here.
Of course I might be wrong, and sorry to sound like a smart ass, I'm almost qualified as a doctor but I speak mostly from experience - I've had 2 serious knee surgeries myself and I've recovered 100% from both, so it's not the end of the world but it needs time!