I recently installed Ubuntu on a PC that also ran windows and it looks like it failed to detect windows. This post describes the steps to fix this issue.
Initially I installed Windows 10 to the first partition on the hard drive and left a space to install Ubuntu. During the installation of Ubuntu I decided to install GRUB to manage the operating systems.
GRUB is a bootloader which detects what operating systems are installed and lets you pick which to run. However when I checked what operating systems GRUB detected it didn’t find my Windows 10 install. It only let me choose to boot into Ubuntu.
Updating GRUB with new operating systems
So that GRUB would find my Windows partition I needed to force it to rescan the partitions. This would then hopefully detect that I have a windows 10 bootable partition. To rescan the partitions GRUB found I ran:
After running this I rebooted the machine and found Windows 10 listed.
Once this had been done I was able to boot into Windows 10 as expected. Normally GRUB will detect windows first time but for unknown reasons this failed the first time around.