To recover GRUB, you’ll need the OS ISO image, in this case CentOS 7, and boot from it.
From the ISO menu, choose Trobleshooting -> Rescue a CentOS system and then 1) Continue; the system will be mounted under /mnt/sysimage.
To make your system the root environment run:
chroot /mnt/sysimage
Identify GRUB installation:
ls /sbin | grep grub
Install GRUB:
/sbin/grub2-install /dev/sdX
Exit and reboot.