'Outlook cannot connect to server' means Outlook can't reach the mail server at all. Here's what causes it and how to restore your connection.
Your company's server settings or credentials changed — this is an IT issue, not something you can fix alone.
Next step: Contact your IT helpdesk. They'll need to verify your account status and may need to update the server settings on your device.
Microsoft's servers may be having a temporary outage, or your local credentials expired.
Next step: Check status.office.com for any active incidents. If status is OK, open Outlook → File → Account Settings → your account → Test. Sign out and back in if needed.
Your email provider may have changed security requirements (common after enabling two-factor auth).
Next step: Check your provider's current IMAP/POP settings page. If 2FA is enabled, you likely need an 'app password' — most providers generate these in account security settings.