I know this will happen to me again at a bad moment so I'm putting it here on my blog and hoping I can find it in my future hour of need ...

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!