I have a dual-boot machine, with Ubuntu 14.04 and Windows. Recently, my Windows did an automatic update from 8.1 to 10. After this, my machine no longer booted to the grub menu; instead, it booted straight to Windows.
So, in Windows, I downloaded boot-repair, and burned the ISO to USB. I then booted from the USB, and ran the boot-repair utility, based on the "recommended repair" option. This then told me that it had been successfully repaired.
However, after rebooting, my machine still boots straight to Windows, and does not show me the grub menu. I have tried the accepted answer at Windows 10 upgrade kills grub and boot-repair doesn't help, but this does not help -- my machine still boots straight to Windows.
The details from boot-repair can be found here: http://paste.ubuntu.com/15495899/