Dual Boot - Ubuntu disappeared
There are a few options to repair booting into Ubuntu. First, be sure Windows is shut down without hiberfil.sys or Fast Startup or the HDD will not be accessible in write mode.
Download a version of Ubuntu and create installation media. Boot from the USB drive (or DVD) to "try Ubuntu", then install boot-repair and try it.
If boot-repair did not resolve the issue, then you may need to manually change the UEFI boot order.
If all else fails, using Ubuntu boot media, back up the UEFI boot loaders and then replace the UEFI boot file in
/boot/efi/EFI/Microsoft/bootmgfw.efiwith that in a previously-working Ubuntu folder, renamed to bootmgfw.efi. N.B. this is deceptive, if someone needs to troubleshoot bootup, but I admit to doing this to resolve an issue where even uninstalling Windows and reinstalling Ubuntu did not work.
I had the same problem - dual boot Windows 10/Lubuntu computer which would no longer show the GRUB dual boot menu after the Spring 2018 Windows 10 upgrade.
My Dell system has an F12 (Boot Order) key that saved me a lot of potential work: I kept pressing F12 during startup, and the computer went into its own boot menu with separate entries for Windows and Lubuntu.
Once I booted into Lubuntu, I opened a Terminal and entered "sudo grub-install /dev/sda" (both operating systems are on sda on my computer). Grub dual boot menu was reinstalled and dual boot is working fine again. Total time about 3 minutes.
Hope your system has the same options available.