Microsoft investigates Windows 11 January 2026 update causing boot failures on some PCs

Microsoft has confirmed that a limited number of Windows 11 users are experiencing critical boot failures after installing the January 2026 security update released on January 13. According to an official online bulletin, affected systems may fail to start and instead display a black screen accompanied by the stop code “UNMOUNTABLE_BOOT_VOLUME,” leaving users unable to complete the startup process without manual intervention.

The issue appears to primarily affect physical machines running Windows 11 versions 24H2 and 25H2. While Microsoft states that only a small number of reports have been received, the severity of the problem has raised concern, as impacted devices require recovery through the Windows Recovery Environment and the removal of the latest security update to restore functionality. At the time of reporting, Microsoft has not identified the root cause of the boot failures, nor has it released a fix or workaround beyond manual recovery.

This latest problem adds to a growing list of issues associated with January 2026’s Patch Tuesday updates. Earlier reports indicated that Windows 11 version 23H2 systems were unable to shut down or hibernate properly, while newer versions encountered Remote Desktop sign-in failures. Additional bugs also disrupted cloud-based applications such as Outlook, OneDrive, and Dropbox, prompting Microsoft to release two emergency out-of-band updates to address those problems.

With boot failures now added to the list, industry observers expect Microsoft may need to issue yet another emergency update. The situation has sparked criticism over the quality assurance process behind recent Windows updates, with commentators suggesting that Microsoft reassess its development and testing practices to prevent similarly disruptive issues in the future.