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UNMOUNTABLE_BOOT_VOLUME

Windows could not open the drive it boots from. Usually file system damage, sometimes a failing drive.

How to fix it

1. Boot to Advanced startup (or a Windows USB > Repair your computer) > Troubleshoot > Command Prompt.
2. Run: chkdsk C: /f (the letter may be different there; use dir D: and so on to find Windows).
3. Then run Startup Repair.

Is it serious?

chkdsk fixes many cases. Repeat failures mean the drive is dying: copy your files off first.