From: https://www.dell.com/support/article/us/en/04/sln300987/how-to-repair-the-efi-bootloader-on-a-gpt-hdd-for-windows-7-8-8-1-and-10-on-your-dell-pc?lang=en

This article provides information about how to resolve an issue with the EFI Bootloader not booting correctly on a GPT Hard Disk Drive for a number of Windows Operating Systems.

Table of Contents:
Introduction
How to repair your EFI Bootloader
How to repair your MBR (Master Boot Record)

From: https://blog.d0zingcat.xyz/2015/09/28/Windows/How%20to%20repair%20the%20EFI%20Bootloader%20in%20Windows%2010/

NOTE: have succesfully used Repair on Windows 2018 version install disk.

The Fix
1. Assign the Drive Letter
2. Repair the Boot Record
3. Rebuild the BCD store

From: https://forums.tomshardware.com/threads/windows-10-wont-boot-bootmgr-is-missing-bootrec-fixboot-access-denied.3297648/

Here are commands to fix bootloader:
diskpart
select disk 0
select partition 2
assign letter=g
exit
bcdboot c:\windows /s g:

From: https://pureinfotech.com/restore-registry-backup-windows-10/

Type the following command to move inside the RegBack, which contains a 
backup of the Registry, and press Enter:

cd c;\windows\system32\config\RegBack

http://msdn.microsoft.com/en-us/library/windows/hardware/gg463059.aspx

Microsoft has completely reengineered the boot environment for Microsoft Windows Vista to address the increasing complexity and diversity of modern hardware and firmware.

http://www.pronetworks.org/forums/how-to-edit-the-windows-vista-boot-menu-options-t79102.html

  bcdedit /set {5189b25c-5558-4bf2-bca4-289b11bd29e2} description "Windows Vista Build 5270 x64"

Changes the text of the boot menu line for any other Vista installation. One must use the GUID for that particular installation as shown when one runs the bcdedit or bcdedit /enum all command

Correcting changes to the Partition/Disk structure

Where a partition or a hard drive has been added or removed and has caused the partition/disk structure to change, this can be corrected by running these commands in the order shown:

  X:\>X:\boot\fixntfs.exe -lh -all (Where X: is the drive/partition on which the folder "boot" is to be found)

  bcdedit /set {5189b25c-5558-4bf2-bca4-289b11bd29e2} device partition=X:

Changes boot partition of the OS whose GUID is indicated. (Where X: is new drive/partition required). Must be used together with the osdevice command below

  bcdedit /set {5189b25c-5558-4bf2-bca4-289b11bd29e2} osdevice partition=X:

Changes boot partition of the OS whose GUID is indicated. (Where X: is new drive/partition required). Must be used together with the device command above

From: https://technet.microsoft.com/en-us/library/hh825146.aspx

====== This documentation is archived and is not being maintained. Sample: Configure BIOS/MBR-Based Hard Disk Partitions by Using Windows PE and DiskPart mbr recovery partition requirements to boot

From: https://technet.microsoft.com/en-us/library/hh825677.aspx

From: https://technet.microsoft.com/en-us/library/hh825089.aspx

From: https://msdn.microsoft.com/en-us/windows/hardware/commercialize/manufacture/desktop/configure-biosmbr-based-hard-drive-partitions

From: http://www.partition-recovery.com/

Active@ Partition Recovery is a freeware toolkit that helps you to recover deleted and damaged logical drives and partitions within DOS, Windows, WinPE (recovery boot disk) & Linux (recovery LiveCD) environments.