Verify your username and password are correct.
You can also try to reinstall the app.
Try a manual refresh of your data. This will force a full resynchronization of data from the server. If that doesn't work, try stopping the app or even reinstalling the app. You can stop the app by:
Your school controls what areas of the app available to you. It's common for schools to turn off all or parts of the app over summer break, during other school breaks, and during grading periods.
Contact your school if you have questions about what's turned on and off.