My work machine is a Windows PC, with dual boot to the Ubuntu partition that I actually use. Sometimes, when booting Windows, it "repairs" its disks and removes my grub menu, booting straight into Windows without showing me the grub menu.
To fix this: interrupt the startup, choose the boot device and pick the Ubuntu partition from the list.
Once booted, you can
boot-repair to sort out grub - I also travel with a boot disk, just in case!