Re: Forced Changes
When you have a boot manager for dual booting Windows 10 or Linux, it gets disabled so you can only boot into Windows 10.
I've heard of this anecdotally, but I've never seen it in the limited contact I have had with Windows 10 in dual-boot setups. I'm a proponent of dual-boot setups, and I have them on all of my PCs that have enough space for more than one OS (in other words, all but my cheapy laptop with a 32GB eMMC "hard drive" that can't be upgraded).
Some of my Windows-using compatriots on other forums are fearful about dual-boot because of these reports, and are either avoiding it or are pondering unusual configurations like external a/b power switches for internal hard drives... but as I see it, if you're running Windows 10, it's a disaster waiting to happen one way or another, so you had better have a plan "B" even if you aren't dual booting. Backups (full system images) are essential.
That said, repairing a GRUB dual-boot setup is easy. Grab an ISO for the Ultimate Boot CD (UBCD) and write it to the USB drive (or an actual disc if you like) of your choice. Boot the PC that needs repair from the USB drive/disc, and select HDD, then Boot Management from the menu. One of the options will be Super GRUB2 Disc... select that, then follow the menus (I can't remember the exact options) to list all of the operating systems on the hard drive. Select your Linux from the list (your choice of whatever kernels you have installed), and let it go... it will take longer than usual, but it will (hopefully) boot your Linux, from which you can then open a terminal window and type (at least in Mint)
sudo grub-install /dev/sda
or sdb, sdc, etc., depending on where you want the bootloader. Most of the time, it will be sda.
That should do it... it always has for me (when I have messed up GRUB by other means... it's never been Windows that did it, though I don't use 10 either).