I'd never heard of this BSOD before so I looked it up, there are a multitude of possible causes so you'll have to go through them one at a time, my guess is that you won't do that so expect some more of this BSOD in the future.
The Windows error code 0xc000021a, also known as STATUS_SYSTEM_PROCESS_TERMINATED, indicates that a critical system process like Winlogon or CSRSS has failed, causing the operating system to halt. This can lead to a Blue Screen of Death (BSOD) and prevent the computer from booting properly.
Possible Causes:
Corrupted system files: This is a common cause, especially after a failed Windows update or a corrupted hard drive.
Incompatible software: An incompatible third-party program can sometimes interfere with critical system processes.
Failed Windows upgrade: An incomplete or failed upgrade can leave the system in an unstable state.
Incomplete installation: If a software program was not installed correctly, it could lead to this error.
Backup issues: If a backup program did not correctly restore files, it could lead to mismatched system files.
Malware: In some cases, malware can also interfere with system processes.
Hardware problems: While less common, hardware issues like a failing hard drive can also cause this error.
Troubleshooting Steps:
Boot into Safe Mode: Try booting into Safe Mode to see if the issue persists. If it doesn't, it suggests that a third-party application or driver might be causing the problem.
Run System File Checker (SFC): This tool can scan for and repair corrupted system files.
Update drivers: Make sure your drivers are up-to-date, especially graphics drivers, as they can sometimes cause issues.
Run Windows Defender Antivirus scan: Scan your system for malware, as it could be interfering with system processes.
Check disk errors: Use CHKDSK to check for errors on your hard drive.
Uninstall conflicting software: If you recently installed a new program, try uninstalling it to see if that resolves the issue.
Perform a System Restore: If you have a recent system restore point, restore your system to that point.
Reinstall Windows: If all else fails, you may need to reinstall Windows.
Consider using DISM: The Deployment Image Servicing and Management tool can be used to repair the Windows image.
Use the bcdedit command: This command can help rebuild the Boot Configuration Data (BCD), which is used by the bootloader.
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