Ditto what Gordon said re. consulting a professional. Unless you are sure what the problem is (i.e., a longstanding problem that's already been diagnosed and flairs up occasionally), I would see an orthopedic specialist. I've had various knee problems over the years, including one minor and one major surgery. Certain types of ligament injuries can get worse without treatment; for example, a torn ligament can keep getting caught in the joint and re-tear. Also, the blood supply to a damaged ligament can change quickly, and if it has to be repaired, there's a better prognosis for repairing it sooner rather than later.

I hope this is just a minor injury for you, but if you're unable to tolerate (pain) or flex the knee enough to complete a pedal rotation on a stationary bike, you need to see someone....soon. Cycling is a knee friendly exercise and even with serious injuries and surgeries, the physical therapists have you pedaling from the outset of rehab.

I would take some antiinflamatories and call the orthopod. My preference is to see Sports Medicine specialists for these kinds of problems, 'cause they're used to dealing with active people who want specific kinds of advice/rehab to match their sport. Most major medical centers have Sports Medicine clinics.

Best of luck to you
Mary Ann