Microsoft has acknowledged a serious issue with the Windows 11 January 2026 Update, where some PCs fail to boot after installation. This problem, identified by the stop code UNMOUNTABLE_BOOT_VOLUME, has left affected devices unable to start, requiring manual recovery. Microsoft notes that the issue is limited to specific hardware configurations but has not yet pinpointed the exact cause. Users experiencing this problem are advised to use Windows Recovery or reinstall Windows via an ISO if WinRE fails to resolve the issue.

The affected update, KB5074109, is a mandatory security update for Windows 11 25H2 and 24H2, addressing over a hundred known security vulnerabilities. This update was released on January 13, 2026, and has been automatically installed on compatible systems. Microsoft is collecting data from affected users and IT admins to better understand and resolve the issue. In addition to boot problems, the update has caused other issues, such as crashes in Outlook Classic and problems with sleep mode on older PCs.

Users can check if they have the January 2026 Update by navigating to Settings > System > About > Specification and looking for the build number 26200.7623. Microsoft has issued an emergency update, KB5078127, to address some of the problems caused by the January 2026 Update, including fixes for Outlook POP, PST crashes, and unresponsive apps. Despite these challenges, Microsoft assures that the affected user base is limited, and most Windows 11 users should not experience these issues.

The incident underscores the ongoing challenges of rolling out major software updates, particularly for an operating system as widely used as Windows 11. As Microsoft continues to gather data and work on a resolution, users are advised to stay vigilant and report any issues they encounter. The company’s transparency in acknowledging these problems and providing workarounds is a step toward maintaining user trust during such trying times.