Make sure that your sample.fxml is in the src/main/resources/ directory (or a subdirectory). Then you can access the file like this:
Parent root = FXMLLoader.load(getClass().getClassLoader().getResource("sample.fxml"));
Explanation:
During the compile phase all resources and classes get copied to target/classes/. So your fxml file resides in this directory and your class in a subdirectory regarding its package name. If you call getClass().getResource("sample.fxml"); the file will be searched relative to the class file which will be this directory: target/classes/sample/.
Calling .getResource() on the classloader sets the relative search path to target/classes/ and therefore your file gets found.
P.S. You could also write:
Parent root = FXMLLoader.load(getClass().getResource("/sample.fxml"));