Start with the right checks.
Working Offline can be a selected mode rather than an internet outage. Check the status message and compare access in a browser.
What to do next.
Try an ordinary website to confirm connectivity.
In classic Outlook, review the status bar and the Send/Receive tab.
If Work Offline is selected unintentionally, toggle it and check the status again.
Test with one harmless message rather than resending everything at once.
Check the result.
Confirm the test leaves the Outbox and reaches the recipient. Then check whether the original pending messages need attention.
Stop and get help if…
Outlook says Disconnected or Trying to connect after the mode is corrected, or webmail also fails. Record the exact status instead of repeatedly toggling settings.