Windows 11 some computers cannot start

The January 2026 Patch Tuesday for Windows 11 continues to cause problems for users. After reports of frozen applications, problematic applications, and the unprecedented inability to terminate of computers, which are said to have now been solved after the second OOB emergency update (KB5078127, KB5078132), users are experiencing a much more serious issue where their computers simply refuse to boot, displaying the “UNMOUNTABLE_BOOT_VOLUME” error.

The problem is as serious as it sounds, and Microsoft has already confirmed it. In the Admin Center, the company issued a warning that it had received a “limited number of reports” where computers were unable to start and were displaying the “UNMOUNTABLE_BOOT_VOLUME” error after applying the KB5074109 update from January 13 onwards.

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How Microsoft describes it:

Microsoft has received a limited number of reports of an issue where devices fail to boot with an error code “UNMOUNTABLE_BOOT_VOLUME” after installing the Windows January 2026 security update (the initial KBs listed above), which was released on January 13, 2026. Affected devices display a black screen with the message “Your device encountered a problem and needs to restart. You can restart.” At this point, the device cannot complete booting and requires manual recovery steps.

At this point, those experiencing the issue should follow “manual recovery steps” to get their PCs back up and running. Microsoft doesn’t say what those steps are, though. The company only says the issue affects local PCs on Windows 11 builds 25H2 and 24H2.

The company is investigating the issue and will share more information as it becomes available.

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