If you have a Windows re-install disc you can use dism to re-write your Windows bootloader, however this will Disable your ability to boot Ubuntu and require you to re-install ubuntu.
In WINDOWS command prompt type:
"bcdboot C:\Windows /s E: /f UEFI" (C is your windows location, change accordingly. E is your UEFI partition, change accordingly as your drive letters may differ). To see what drive letters to use type "notepad" into the command prompt then "File/Open" to browse so you can see what letters are in use. If your UEFI parition isn't mounted which it likely won't be then you will need to do so by in command prompt typing "diskpart", "list disk", "select disk 0" (or whatever disk it's on), "select partition 0" (or whatever partition it is), "assign letter=q" (or whatever letter you want to use that is currently free).
If you don't have a windows re-install disc you can always download the Windows 8.1 - Enterprise X86-64 iso file. Registration is required.