Hypothetically, if you were to travel back in time, no matter what you do or don't do, you will have changed something. Your mere existence there would be different from what originally happened. So, whatever changes you make (however small) will affect the timeline you are in. With the timeline altered, the event in which you originally went back in time will also be slightly (or drastically) changed. If it did not affect the fact that you traveled back in time, then it will have affected some reason, or means of you traveling back in time. Thus, when you traveled back in this timeline, you will change the timeline some other way.
This process will continue repeatedly until it comes to the right circumstances that you never traveled back in time anyway.
The preceding hypothetical situation would happen that way if there is only one timeline. If there is more then one timeline, then upon traveling back in time, you will also shift into a different dimension/timeline. One that has the occurrences of what happens due to the changes you would have made because of traveling back in time.