If you're having issues opening Outlook and are using an Office 365 account, your issue might be improperly-formatted credentials stored in Windows Credential Manager. Here's how to remove them: 

Credential Manager lets you view and delete your saved credentials for signing in to websites, connected applications, and networks.

  • To open Credential Manager, type credential manager in the search box on the taskbar and select Credential Manager Control panel
  • Select Web Credentials or Windows Credentials to access the credentials you want to manage.
  • In the Windows Credentials and Web Credentials section, remove any stored credentials referencing the Office 365, ms.outlook email address or Windows login