Situation: I had Ubuntu 18.10 successfully dual-booting with Windows 10. Tried to do_release_upgrade to 19.04 but it couldn't complete, probably because I had a million *-desktop packages installed and they'd trodden all over each other.
So I decided to install a fresh 19.04, using all defaults for everything. Install went smoothly but on reboot, there was no GRUB menu, just straight into Ubuntu.
After MUCH experimenting, it seems that the problem was that the default install was creating an "efi" partition which was effing it up somehow. The fix was to re-run the install, but instead of accepting the option to delete the previous Ubuntu and install over it, choose to "do something else", and manually the pick the partition to nuke and recreate. Do not create an efi partition.
It will piss and moan about not having an efi partition, but ignore it. It's fine, it will boot fine. Once you're up and running, pop a terminal and do:
sudo update-grub
It will detect all your bootable OSes and add them to the list (this is what it fails to do when there's an efi partition).
If you want to change anything else about GRUB, edit /etc/defaults/grub and run sudo update-grub. A few of my favourite changes:
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
GRUB_TIMEOUT=10