Here are some ways to fix it:
* "You walked past the girl." (This is the most likely intended meaning.)
* "Did you walk past the girl?" (This is a question)
* "You walked past her." (This is a more concise way to say it, assuming the listener knows who "her" refers to)
It's important to be clear about who is performing the action (walking past) and who the object of the action is (the girl).