https://www.bleepingcomputer.com/news/microsoft/microsoft-outage-caused-by-overloaded-azure-dns-servers/

Microsoft has revealed that Thursday's worldwide outage was caused by a code defect that allowed the Azure DNS service to become overwhelmed and not respond to DNS queries.

At approximately 5:21 PM EST on Thursday, Microsoft experienced a global outage that prevented users from accessing or signing into numerous services, including Xbox Live, Microsoft Office, SharePoint Online, Microsoft Intune, Dynamics 365, Microsoft Teams, Skype, Exchange Online, OneDrive, Yammer, Power BI, Power Apps, OneNote, Microsoft Managed Desktop, and Microsoft Streams.