Step 1 — Startup Repair

Boot from a Windows 10/11 USB → Troubleshoot → Advanced Options → Startup Repair. This fixes most BCD and bootloader corruption automatically.

Step 2 — Manual BCD repair

From Advanced Options → Command Prompt:

bootrec /fixmbr
bootrec /fixboot
bootrec /scanos
bootrec /rebuildbcd

Step 3 — SFC and DISM offline

sfc /scannow /offbootdir=C: /offwindir=C:Windows
DISM /Image:C: /Cleanup-Image /RestoreHealth

Step 4 — In-place repair upgrade

Boot Windows normally (if possible) from the install USB → Upgrade this PC → Keep files and apps. Replaces all system files without data loss.

Last resort — fresh install

Back up C:UsersYourName to external drive first. Full reinstall takes 20 minutes from USB.