Often, students and their families have a string break on their French horn, making it impossible to use the horn properly.
Although you can take it to a local music shop to be repaired, you may want to consider repairing it yourself. It takes a bit of time the first time you do it (I would plan on about 30-60 minutes if you've never done it before). You also need string and a few tools.
However, once you've learned how to do it and have all the tools and supplies you need, it will take you only 5-10 minutes in the future, and will save you a trip to the repair shop.
This youtube video posted by a band director provides a great tutorial on how to do the necessary repair. You can buy the string you need for the repair online, including at Amazon.