If you've been running for 10 years, but just recently developed knee problems after several weeks of running on a treadmill, I would be inclined to blame the treadmill, especially if you've been doing most of your previous running outdoors. For example, you may think that your form is fine on the treadmill, but your footstrike and overall biomechanics may actually be just a bit different than outdoors. After a few weeks, even a small amount of misalignment could lead to injuries. It might be a good idea to have another experienced runner watch you run on the treadmill - they may be able to point out potential problems.
In my experience, X-rays are quite good at diagnosing structural problems, but not at diagnosing or distinguishing most soft tissue injuries. Before I had biomechanical problems identified and resolved nearly 20 years ago, I was having recurrent medial knee injuries. Several times the doctors thought I had torn cartilage, judging from external symptoms, and warned me about possible impending knee surgery. Fortunately every time it turned out to be some sort of inflammation, which will eventually heal, either on its own or with the help of a cortisone shot (which I would not recommend except on a very occasional basis to break an inflammatory cycle). In one particularly severe case I did have an MRI, and it indeed showed inflammation rather than a tear.
I would also view the bike as cross-training to temporarily replace running while you heal (and to intersperse with running, if you wish, once you've healed). At this stage I would definitely not accept a diagnosis of "not being able to run again".