Sometimes you break your Windows Boot Manager when you set your PC to a dualboot configuration.
In some case, start-up Repair won't help you because files in EFI partition are missing.
If the Windows system partition is untouched, you can copy EFI files from the partition.
Boot with Windows 10 installation media. Choose Repair your computer. Start command prompt.
diskpart
list vol
If the windows system partition is already mounted mounted, skip the assignation process below.
list disk
Choose the disk in which Windows 10 is installed.
select disk XX
list partition
Choose the partition of Windows system.
select partition XX
assign letter=C
Now we'll mount EFI partition. You can determine the EFI partition like what we did. Assign to the letter that is not used, like B:.
mkdir B:\EFI\Microsoft\Boot
xcopy /s C:\Windows\Boot\EFI\*.* B:\EFI\Microsoft\Boot
Now you can use startu-up repair from repair option. You can still boot Linux or other operating systems by choosing EFI files. Automatic boot menu generation should work when you install OS other than Windows..
How to Restore Deleted EFI System Partition in Windows 10? | Windows OS Hub