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CentOS: reinstall GRUB2 from the CentOS 7 installation disk

Boot from the CentOS 7 installation disk.

Select "Troubleshooting".

Select "Rescue a CentOS Linux system".

Select "3) Skip to shell".

Show the disks partitioning:

fdisk -l

Identify the system partition. In this example, it is "/dev/sda1".

Mount the system partition under "/mnt/sysimage":

mount /dev/sda1 /mnt/sysimage

Reinstall the GRUB2 boot loader:

grub2-install --root-directory=/mnt/sysimage /dev/sda

Exit and reboot.