Option 1: Login to Office 365 Portal & change your password & try to sign in with new password on Outlook
https://account.activedirectory.windowsazure.com/ChangePassword.aspx?BrandContextID=O365
Option 2:
- Open Control Panel à Credential Manager à Windows Credentials à Remove all saved credentials of Office365
- Open Outlook à Put new credentials
Option 3: Edit you System Registry (Note: Before making any changes take a backup of your registry)
HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity
Registry Name : EnableADAL
Type : REG_DWORD
Value : 0
Article for reference : https://docs.microsoft.com/en-us/office365/admin/security-and-compliance/enable-modern-authentication?view=o365-worldwide