I use an old computer for testing purposes at home, one problem of grub is when you install a windows system it removes grub and you can’t access to your Centos 7 system.
reinstall grub is really easy.
Boot your computer from a dvd o flash memory with centos 7 install disk
select troubleshooting menu
Select rescue a Centos system
when boot ask you how to mount a system select continue and it gives you access to a shell.
grub2-install –root-directory=/mnt/sysimage/ /dev/sda
a then reboot