Gfilui

I've been trying to fix this issue for the past few hours now. I had kubuntu 17.10 installed before updating to 18.04, and before that ubuntu 16.04, and both recognized windows 10 in the grub menu, but after installing Kubuntu 18.04, at first the grub menu didn't appear at all, but then I did something in a file that made it appear, however windows 10 did not appear as an option in the grub menu.

I tried running sudo os-prober but nothing appears in the terminal, then I ran sudo update-grub and it only shows

Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.15.0-20-generic
Found initrd image: /boot/initrd.img-4.15.0-20-generic
Adding boot menu entry for EFI firmware configuration
done

It may have something to do with when I installed Kubuntu 18.04, as I made an EFI partition alongside /swap and the root partition, something I've never done before when I installed previous Linux distros.

I also tried adding a menuentry in the 40_custom file for windows 10, but that didn't work.

I also ran boot-repair, and this is the result:

GPT detected. Please create a BIOS-Boot partition (>1MB, unformatted filesystem, bios_grub flag). This can be performed via tools such as Gparted. Then try again. Alternatively, you can retry after activating the [Separate /boot/efi partition:] option.

I'm thinking of installing Kubuntu again in the hopes of it working again, should I do that?

Edit2: I just reinstalled Kubuntu 17.10 over 18.04, typed

sudo update-grub

And everything is working again, windows 10 is an option in the grub menu. Thank you @kishea and @oldfred for trying to help me out. I'll wait for a few months until 18.04 stabilizes or something and download it then.

Am Sure You Need to boot into windows.
But to restore the boot menu, try boot-repair utility

sudo add-apt-repository ppa:yannubuntu/boot-repair

then

sudo apt-get update && sudo apt-get install boot repair && boot-repair

After running the utilty. view the log file to confirm the windows installation exists.

if all is fine you must have all your operating systems in the grub menu

if this does not work,

1. press 'c' on the grub prompt


2. type ls command to view the partitions



3. type insmod ntfs



4. 
   

   mark the listed partitions (hd0,msdos1) or (hd0,msdos2) ... or (hd1,gpt1)

   
   
   

   set root=(hdX,msdosX)
   chainloader +1
   boot

   
   

if it fails reboot and repeat the process steps above changing the (hdX,msdosX) to each of values you get in ls command.