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.
On a dualboot system running both Windows 10 and i.e. Debian, Windows 10 tends to overwrite the UEFI boot ROM. This means the GRUB UEFI entry dissapears, and you won't be able to boot to it. To fix this, you want to find a way to boot to it anyway, and add GRUB back into the boot ROM.
The following solution works if your motherboard comes with a built-in EUFI loader:
map command to list all filesystems.fs5:)ls to see list the root directory.fs*:EFI/debiangrub64.efiefibootmgr and verify that your os (i.e. Debian) is not in the listing.sudo grub-install /dev/sdc. (Replace /dev/sdc with whatever disk your os is on.)sudo update-grub and then efibootmgr again, to check if Debian is now listed in the bootsection.