If you find that some Outlook clients stopped connecting to the Exchange Online mailbox but OWA and ActiveSync is still connecting then check if modern authentication has been disabled for your O365 tenant
To check run:
Get-OrganizationConfig | Format-Table Name,OAuth* -Auto
If you find it is set to False and all your Outlook clients are 2013 or newer then enable it by running:
Set-OrganizationConfig -OAuth2ClientProfileEnabled $true
Your disconnected users should start connecting within a few minutes.