No GRUB after installing Ubuntu beside windows 10

You have to disable fast startup in Windows:

1. Go to Control panel -> All control panel items -> Power options -> Choose what the powers button do
2. Click on Change settings that are currently unavailable
3. Uncheck Turn on fast startup
4. Uncheck Hibernate
5. Restart entering Bios/UEFI
6. Move Ubuntu on top of Windows
7. Save and restart

Now everything should work!

1. In Windows, go to the menu.
2. Search for Command Prompt, right click on it to run it as administrator.
3. In here, copy paste the command: bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi
4. Restart and you’ll be welcomed by the familiar Grub screen.

This one worked for me. Source & credits to https://itsfoss.com/no-grub-windows-linux/ Abhishek Prakash

if you an a newer computer the bios may be defaulting to the windows boot manager, go to bios and make sure the ubuntu uefi is the first boot option. This should correct the problem.