Answer Description
GRUB (GRand Unified Bootloader) is an open source technology commonly used to allow multiple operating systems to boot on a single machine. GRUB is commonly used as the default boot loader for Linux, but it is compatible with Windows also. When Windows 10 was installed the GRUB was replaced or corrupted and using the Linux installation CD to restore it is your best option.
Wikipedia
GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a user the choice to boot one of multiple operating systems installed on a computer or select a specific kernel configuration available on a particular operating system's partitions GNU GRUB was developed from a package called the Grand Unified Bootloader (a play on Grand Unified Theory) It is predominantly used for Unix-like systems The GNU operating system uses GNU GRUB as its boot loader, as do most Linux distributions and the Solaris operating system on x86 systems, starting with the Solaris 10 1/06 release
GNU_GRUB - Wikipedia, the free encyclopedia