In the case it's grub, get a live cd with any linux version on it.
(try lubuntu or something).
when in lubuntu, sudo su - root
mount /dev/sdyX /mnt/kali (y being either a or b, X being the number your root partition is on of your Kali install).
grub-installer or grub-install, dunno, auto-finish with tab
might be that its grub-install /dev/sdy (same y as ealrier (probably its a).