When something breaks, first check whether it is Microsoft:

1. Admins: the Microsoft 365 admin center, under Health > Service health. Incidents that affect your tenant appear only there.
2. Everyone: status.cloud.microsoft, Microsoft's public status page. It lists only widespread incidents.
3. The service status board shows Microsoft 365 next to Azure, AWS, Google Workspace and others, in case the problem is bigger than Microsoft.

Sign-in errors: a code starting with AADSTS (AADSTS50126, for example) comes from Entra ID, and the number says why the sign-in failed. The error code library explains the common ones. If an Office app just says "Something went wrong", start with this Cache article.

When you post, include which app, and whether it is the desktop, web or mobile version; the exact error; whether it affects one user or everyone; and what changed recently. Leave out tenant names, domains and email addresses.