I am dual-booting Windows 10 and Ubuntu, I had an issue with GRUB breaking and I fixed it, but GRUB is missing from the boot menu, yet still works.
How it broke and how I fixed it: Earlier yesterday I installed some kind of software update on Ubuntu 16.04 and it asked me to restart so I did. I remember it then booted into Windows without going through GRUB, but I didn't think anything at the time and turned off for the night. Today I tried to go into Ubuntu, but GRUB wouldn't boot, so I tried to select it from the Laptop's boot menu and it just booted Windows. I then used boot-repair from a Live Ubuntu USB and did the recommend repair (http://paste.ubuntu.com/25865166/) and GRUB failed to boot (Something about missing files I think, the error message disappeared too fast for me to read) and I got a padlock with something about secure boot (The only thing I could boot to was the Live USB).
I then figured out how to disable secure boot from the BIOS and now GRUB works fine (Although there are a lot more boot options now, but I think Grub Boot menu has too many entries can help with that.
The reason I'm posting this is because before GRUB appeared in my laptop boot menu, but now its not there (Some Windows option is on the top now), yet it can still boot into GRUB (And does by default).
I don't really know what I'm doing and GRUB breaking really stressed me out so I don't want to touch it again for a while, but I can't understand why GRUB is booting if the BIOS can't detect it? Any ideas?