I know that this is a common problem but my situation is a bit different. I had a perfectly working Linux Mint 17.2 and Windows 10 in dual boot with grub menu showing and giving me options on what to boot for weeks now, but then I wanted to boot into BIOS but I forgot what key will let me go into the BIOS so I just randomly pressed all the functions keys and something appeared on the screen but it was too fast for me to read then it went into BIOS. I didn't touch anything in the BIOS and after exiting it booted directly into Windows 10 and I haven't booted into Linux ever since.
Things I've tried:
- I tried changing the boot options(F12) but ubuntu wasn't listed there
- Turned off fast boot and hibernate in Windows 10
- bcdedit in windows
- Manually reinstalling grub by following the answer here https://askubuntu.com/questions/683108/grub-menu-not-showing-at-boot
- Booted into a live mint 17.2 and did boot-repair
Here is my boot-repair boot info http://paste2.org/pMP8E2WY
I just remembered after exiting bios and as it booted into windows 10 there were updates so maybe that update could have killed my grub.