First of all, don’t worry! We’ll get in touch with both you and the student to find out what happened. We also encourage that you yourself ask the student what went wrong for them and possibly suggest some ways to improve their impression/make amends.
If they are willing to change the rating after that, they have an opportunity to do it. If somehow they don't know they can, feel free to let them know.

More tips on how to improve your rating find here