Microsoft is currently looking into reports of Windows 11 systems failing to start following the January 2026 security updates. The problem specifically impacts physical hardware running versions 24H2 and 25H2, resulting in a blue screen error known as UNMOUNTABLE_BOOT_VOLUME.
Tech support teams have observed that the issue stems from the KB5074109 cumulative update which was distributed earlier this month. While virtual machines appear to be unaffected, users with physical PCs are finding their devices stuck in a reboot loop that requires manual recovery steps to resolve. Microsoft has acknowledged these reports and is working to determine if the crash is a direct regression caused by the new code.
When the error occurs, the operating system displays a standard crash screen notifying the user that the device encountered a problem and needs to restart. However, instead of returning to the desktop after a reboot, the system remains unable to load the boot volume. This has forced administrators to seek alternative recovery methods since the standard automated repair process often fails to bridge the gap.
The investigation was highlighted after independent monitors noticed a spike in complaints from enterprise environments. In response, Microsoft has requested that those experiencing the boot failure submit detailed diagnostic data through the Feedback Hub. They have committed to updating their official documentation once they have confirmed the specific root cause and developed a reliable fix for the impacted versions.
In a separate but simultaneous effort, the company also issued emergency out-of-band updates to address a different bug involving Microsoft Outlook. That particular glitch caused the email client to freeze when data files were hosted on cloud storage platforms like OneDrive. While that fix is already available, the boot volume issue remains under active investigation with no official patch released yet.
Source: Microsoft Investigates Windows 11 Boot Failures After January Updates