If you take a leave of absence as a Northwestern student, it's likely that your NetID will deactivate while you're away. When you are ready to return, follow the Registrar's procedure on reactivating your student status: https://www.registrar.northwestern.edu/registration-graduation/registration/former-students-re-entry.html.
Your registrar’s office’s reactivation of your student status will also automatically reactivate your NetID. After your NetID has been reactivated, your academic dean’s office must obtain an activation code for you by sending a request to servicedesk@northwestern.edu. Your dean’s office will then send you that activation code so you can reset your NetID password. Once you receive it, go to http://www.northwestern.edu/login. Log in using your NetID as the Username, and the activation code as the Password. Then follow the prompts to set your personalized password. For more information about changing and resetting your NetID password, see https://services.northwestern.edu/TDClient/30/Portal/KB/ArticleDet?ID=1399.