|Adding Windows 7 (or 10) to the grub boot options on a Centos 7 system »|
Evidently sometimes Windows 10 will add a partition when upgrading which means grub can't find it's config file.
To fix this you can boot using the Centos 7 install DVD into rescue mode. You need to choose the boot option that matches whatever version of Centos you installed, either legacy boot or EFI,
(for legacy boot)
grub2-mkconfig -o /boot/grub2/grub.cfg
grub2-install /dev/sda (replace the a with whatever drive is your boot drive)