Interrupting the Windows update process (by accident, due to power failure or perhaps hardware failure) may leave the system in a broken non-bootable state. This can manifest by the startup sequence never finishing (getting stick at the spinning circle screen forever) or simply crashing and automatically restarting after a while.
If you assume this to be the case with your system and the usual recovery methods (startup recovery and checkdisk-ing) fail, there may still be some options to use to attempt recovery.
Proceed by starting the system into startup recovery once more, ensure that storage is functional and no corruption exists first (by running chkdsk on the system disk/partition) and then try to repair the Windows installation with:
sfc /scannow /offbootdir=X:\ /offwindir=X:\Windows
Replace X:\ with the actual system partition letter (usually C:\ or D:\ in recovery mode).
If that fails with the following error message: There is a system repair pending which requires a reboot to complete. Restart windows and run sfc again
Run the following command instead:
dism.exe /image:X:\ /cleanup-image /revertpendingactions
This should undo the incomplete update and return the system to a functional state at the next reboot in most instances.
