I had both Mint 17.2 and Win 10 running for over a month, today I booted laptop and it goes directly to Windows. Can any one tell me what happened? I've tried rebooting several times same results, also tried going to boot  it shows the Linux drive but still boots to Win after selecting.This is a Yoga 2 Pro i7 4510-U 8Gb ram 500Gb SSD half drive for Win half for Linux.How can I regain access to the GRUB menu? 
dual boot menu missing
grub2
Seems like your windows bootloader overwrote the default EFI entry to boot into. You can at first check if the grub menu entry is still present in the boot menu, the one that you access pressing the Boot options key when starting the pc. Usually is F10/11/12 or ESC key. If it is there, try to boot into grub bootloader. If it works, you just have to enter the EFI menu (the old BIOS menu), and set the grub option as the first one.If the EFI entry is not present, then you should check if the grubx64.efi file is still present on your EFI system partition. The easy way to do this is booting a live linux distro, and runsudo fdisk -l #locate the EFI system partition, should be around 200MB in FAT32sudo mkdir -p /mnt/espsudo mount /dev/<your ESP partition> /mnt/espthen search in any directory for a grub folder and/or grubx64.efi file. If it is not there, probably you need to reinstall grub. If it is there, you need to create again the menu entry for it. Check efibootmgr or bcfg with EFI shell on how to do this.