Updated
Quick fix
Sign in at microsoft365.com with your full work email. If Outlook isn’t sending or receiving, check your internet, restart Outlook, and make sure it isn’t set to work offline. Your admin can reset passwords and check Microsoft’s service status.
Step by step
Use your full work email
Sign in with your full work email address (for example you@yourbusiness.com) at microsoft365.com or in the app.
Forgot your password?
Use “Forgot my password” if your organization allows self-service reset. Otherwise, your Microsoft 365 administrator can reset it.
Approve the sign-in
If your account uses multi-factor authentication, approve the prompt in the Microsoft Authenticator app or enter the code. Only approve sign-ins you started yourself.
Outlook: check the basics
Check your internet, then close and reopen Outlook. In the classic Outlook app, make sure Work Offline (on the Send / Receive tab) isn’t turned on.
Try Outlook on the web
Sign in at microsoft365.com and open Outlook in the browser. If mail works there, the problem is the Outlook app on that computer.
Check for a service problem
Your administrator can see Microsoft 365 service health in the admin center.