There are a few different ways that you can fix a Windows BCD error, depending on what is causing the error.
If the BCD is missing or corrupt, you can use the Bootrec.exe tool to rebuild the BCD.
If the BCD is corrupt but the boot sector is not, you can use the Bootrec.exe tool to scan for and fix any errors in the boot sector.
If the boot sector is corrupt, you will need to use a bootable repair disc or USB drive to fix the boot sector.