The STOP 0xC000021A error occurs when either Winlogon.exe or Csrss.exe fails. The error occurs due to the below reasons:
a. Mismatched system files have been installed.
b. A backup program that is used to restore a hard disk did not correctly restore files that may have been in use.
c. An incompatible third-party program has been installed.
To be more specific, the system goes to a blue screen because there is some exceptions happened in the kernel (a device drivers, backup software or anti-virus services, etc.), and Windows implements this mechanism: When it detects some errors occur in the kernel, it will kill the box in case some more severe damage happens. Then we get a blue screen or the system reboots (it depends on what the system settings are).
Method 1:
Let’s start the computer by using the last known good configuration. To start the computer by using the last known good configuration, follow the steps mentioned:
a. Restart the computer and tap F8. You will see the Advance Boot options menu.
b. Use the arrow keys to select Last Known Good Configuration, and then press ENTER.
NUM LOCK must be off before the arrow keys on the numeric keypad will function.
c. Use the arrow keys to highlight an operating system, and then press ENTER.
Note:
Choosing the Last Known Good Configuration startup option provides a way to recover from problems such as a newly added driver that may be incorrect for your hardware. However, it does not solve problems that are caused by corrupted or missing drivers or files.