Hey Lowkey,
This is a great opportunity for you to learn about troubleshooting. Here is how:
1. Turn the bars to the side where they stop properly. Look around the head tube and lower triple tree until you find the parts that touch to stop the bars from turning.
2. You can assume that both sides work in a similar way, so turn the bars to the side where they do not stop. What is different from the other side? Is a piece bent or missing? Are the stops not even close to each other before the bars hit the tank? What makes this side behave differently?
Troubleshooting involves learning how something works, and then looking at a situation step by step until you isolate the part that isn't doing what it should be. It is a skill that will help you with all kinds of bike problems. Even if you can't figure out everything, you will be able to ask more specific questions and get better advice from people on the board, and if you go to a bike shop they will be a little less likely to rip you off if you sound like you know something about it.
Regards,
Steve