Since this is a dual boot with Win8 with uefi and what not I am not sure if this would work (it would if you have a pure Linux system whether single or multiboot)
boot from a live usb. Find out where is Ububtu's root (/) partition or just where Ubuntu is if you have no separate root.
(or sudo blkid if the command doesn't show anything)
Mount the partition where your Ubuntu / is (or where Ubuntu is if you have no separate / partition), let's say it is in sda5
sudo mount /dev/sda5 /mnt
sudo grub-install --root-directory=/mnt /dev/sda
Notice the first command involves the partition (/dev/sda5 here) while the second involves the whole drive (/dev/sda here)
Edited: life would be a lot simpler if you get rid of Windows.