For some reason one of my CentOS 7 boxes decided to nuke itself yesterday, when I powered it on it prompted me with the following error: “failed to open \efi\centos\grubx64.efi”.
What I did to fix it is:

• Boot up a rescue USB, mount the required partitions and chroot.
• Add a DNS server:

$ echo "nameserver *.*.*.*" >> /etc/resolv.conf

• Enable networking using ifconfig (in my case it was already enabled).
• Install some additional grub2 modules:

$ yum install grub2-efi-modules

• Reinstall grub, read “/etc/fstab” or “lsblk” command to find it:

$ /sbin/grub2-install /dev/boot/partition

• Reinstall grub:

$ yum reinstall grub-efi shim

• Rebuild grub.cfg file:

$ grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg