Cycling is a great, low-impact way to work out. It, unlike high impact sports like running, football, or tennis, usually doesn't cause muscle strain because of quick movements. But that doesn't mean that there isn't a real risk of injury in cyclists. In fact, most injury in cycling occurs due to poor bike fit or improper riding technique-two things that you are in control of and should work to avoid under all circumstances. The most serious bike injury is a head injury. Wearing a properly fitting bike helmet can help to mitigate the risk of head injury.
The most common injury seen in cyclist is knee pain. Pain may arise from Patellofemoral Pain Syndrome (or what we like to call Runner's Knee). Runner's Knee is pain associated with the irritation of the soft issues at the front of the knee, including strained tendons, muscle overuse, muscle imbalance, and inadequate stretching. It's a fairly common ailment among runners (hence the name) and should be treated by giving your knee a rest for a while and, at the same time evaluating what specific action is causing the pain and avoiding that action. More serious but less common knee injuries that may cause pain and discomfort in cyclists are Chondromalcia (or the deterioration of the underside of the kneecap due to trauma, overuse or poor alignment of the knee joint) and Osteoarthritis of the knee. Osteoarthritis is a very common injury in aging athletes and is degenerative.
Additionally cyclists face the threat of abrasions and road rash if they fall on a hard surface from their bike. Cyclists should avoid falling if at all possible as these sort of tumbles can resort in much more serious injuries such as head injuries, concussions, broken bones, contusions, and more. Paying attention to the road and stopping when you feel you need a rest will help you avoid this.
If you think you have a serious injury, you should schedule an appointment with your doctor. Athletes with minor injuries can be cured using the RICE method as mentioned here many times.