One of those things that happens from time to time on a dual boot system…
Windows breaks grub, usually if I’ve reinstalled windows, this can lead to a unbootable system.
Boot from a Ubuntu (or other distro) live disk or USB stick.
Figure out which your disks and partitions are.
sudo mount /dev/sdXY /mnt
if there is a boot partition
sudo mount /dev/sdUV /mnt/boot sudo grub-install --recheck --root-directory=/mnt /dev/sdZ
When you reboot grub should be back to normal.