If you are getting the 403 Forbidden error in Microsoft Office 365, first open the private mode of your web browser (Incognito mode in Chrome, Private mode in Firefox, and InPrivate mode in Edge). Now, check if you can log in to Microsoft Office 365 or not. If yes, there is some corrupted cache in your web browser. Hence, delete your web browser cache.
If you are not able to log in to Microsoft Office 365, contact your administrator.