Re: Dual booter reinstalled Windows and needs GRUB2 back
If, in fact, Ubuntu is INSIDE your C: drive, do NOT, repeat NOT, reinstall GRUB2. That was a Wubi install, does not use GRUB2, and reinstalling GRUB2 will only make matters worse.
You first need to CONFIRM that this was a Wubi install. Look for a file named root.disk on your C: partition. IF that is there, this is a Wubi install.
Ubuntu 16.04 Mate, Mint 18 Mate; MS Win 8.1, MS Win10 Pro.
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