boot up using a windows 8.1 media (usb/dvd)

Choose advance / command prompt

diskpart
*find the volume to be boot & assign it to letter C where C:\windows locate
list volume
Select volume X (with letter C)
*If X = the correct volume to boot ,
active
*If X does not = the correct volume to boot
remove
select volume Y (the volume to boot)
assign letter=c
active
exit

bcdboot c:\windows /s c:

*maybe

bootrec /fixmbr
bootrec /fixboot
bootrec /scanos\
bootrec /rebuildbcd
bcdboot c:\windows
bcdboot c:\windows /v

repair startup

*other options
bcdedit /set {default} device partition=c:
bcdedit /set {default} osdevice partition=c:
bcdedit /set {bootmgr} device partition=c: