How to fix it

0xC000021A is a serious stop code (a type of blue screen) that Windows raises when a core system process terminates or the system cannot verify a critical process is running. Unlike an app crash, this usually means Windows itself is having trouble starting or running a protected process.

What it means

The code is related to STATUS_ACCESS_VIOLATION at the system level. In practice it most often means one of three things:

  • A critical system process stopped unexpectedly
  • The Boot Configuration Data (BCD) — the list Windows uses to start itself — is damaged or missing
  • A system file that a protected process depends on is corrupt

Where you'll usually see it

  • Immediately after a failed Windows Update
  • After a BIOS/UEFI change (switching between Legacy/CSM and UEFI, changing boot order)
  • After moving a drive or cloning a disk
  • After a sudden power loss mid-update

Common causes

  • Damaged or missing BCD store
  • Corrupt system files (WinSxS / system32)
  • A boot order or mode (UEFI vs Legacy) mismatch
  • A failing drive or a drive whose boot partition is damaged

Quick fixes (try these first)

  1. Boot into the Windows Recovery Environment — hold Shift while clicking Restart, or boot from a Windows USB/install disk and choose Repair your computer → Troubleshoot → Advanced options.
  2. Run Startup Repair — Advanced options → Startup Repair. Let it run to completion and reboot.

Detailed fixes

If Startup Repair doesn't fix it, the usual culprit is a damaged BCD store. From an admin Command Prompt in the Recovery Environment (Troubleshoot → Advanced options → Command Prompt), run:

`` bootrec /fixmbr bootrec /fixboot bootrec /rebuildbcd ``

What these do: bootrec /fixmbr rewrites the Master Boot Record; bootrec /fixboot writes a new boot sector; bootrec /rebuildbcd scans for Windows installations and rebuilds the BCD store. All three are non-destructive to your files — they only touch the boot records. If bootrec /fixboot reports the volume is write-protected, note it and continue with the next step.

If that still fails, repair the system files:

`` sfc /scannow /offbootdir=C:\ /offwindir=C:\Windows ``

What this does: SFC runs offline (the /offbootdir and /offwindir flags point it at the Windows partition) to repair corrupt system files without needing to boot into Windows. It does not delete your personal files or apps.

Then, if the drive is UEFI, rebuild the EFI boot entry:

`` bcdboot C:\Windows /s S: /f UEFI ``

Before running this: confirm the EFI system partition. In the recovery Command Prompt, list the drives with diskpart → list volume. Mount the small (~100–500 MB) FAT32 EFI partition as S: before running bcdboot ... /s S: /f UEFI. Using the wrong drive letter as the target will write the boot entry to the wrong place. This is the one step that benefits from a screenshot of your list volume output if you're unsure.

If you recently changed BIOS/UEFI settings

This is a very common trigger. If you switched between Legacy/CSM and UEFI, or moved the drive's position in the boot order, revert that change first — Windows will often start normally once the boot mode matches how it was installed.

When to suspect hardware

If you also see disk I/O errors, the drive is old, or the same problem follows the drive to another machine, a failing drive or a damaged boot partition is likely. Back up anything reachable (from the recovery environment you can copy files off), and consider replacing the drive.

Related errors

  • 0xC0000005 — Access violation (application level)
  • CRITICAL_PROCESS_DIED — a critical process stopped
  • BAD_SYSTEM_CONFIG_INFO — registry/boot configuration missing