Rebuild Windows 10 Boot Partition

Boot from the install disc (DVD or USB)
Click Repair your computer
Choose Troubleshoot, then Command Prompt
Type these commands in the exact order shown below and press Enter after each command:
DISKPART
SEL DISK 0
LIST VOL
Select the volume number that has FAT32 under the FS column - it is usually volume 1:
SEL VOL 1
“1” is the number of the FAT32 partition printed in Diskpart
 

Assign a unique letter to this partition. The letter must not be already available on your computer, e.g. C: or D: or E:
ASSIGN LETTER=x:
The confirmation message should appear:
“DiskPart successfully assigned the drive letter or mount point”
Type exit to quit the Diskpart utility (step 4):
EXIT
Type the following command, but replace x: with the letter of the partition you chose earlier:
CD /d x:\EFI\Microsoft\Boot\
Type the bootrec command:
BOOTREC /FIXBOOT
Backup your old BCD configuration to create a new one:
REN BCD BCD.Bak
Type this command to replace the BCD, but remember to replace x: with the letter of the FAT32 partition mentioned earlier:
BCDBOOT c:\Windows /l en-us /s x: /f ALL
Remove the media from the drive: DVD or USB
Type EXIT
Hit Enter
Reboot the computer
 

If the EFI boot partition is missing or formatted:
 

EFI partition is usually volume 1 on a disk, about 500 MB.
If there is an unused or NTFS formatted vol 1 of the correct size open diskpart and format it to FAT32:
SEL VOL 1
FORMAT FS=FAT32
Once the disk is done formatting you will have to set the disk ID (GUID) to be an EFI system partition by issuing this command:
SET id=c12a7328-f81f-11d2-ba4b-00a0c93ec93b override
Once that is one you should be able to follow the instructions above.

More information:
https://neosmart.net/wiki/bootrec/#bootrec_RebuildBcd_doesn8217t_work