Je note pour remettre un grub que WS aurait effacé :

(backup old accnt)
Restore Ubuntu-Mate boot menu on a dual boot UEFI overwritten by windows7:

Windows part:
Start+R➡cmd (run as administrator)
mountvol s: /s
s:
cd EFI\Microsoft\boot
copy bootmgfw.efi bootmgfw.efi.old
copy ..\..\EFI\ubuntu\grubx64.efi bootmgfw.efi
mountvol s: /d

reboot
(Now, you have ubuntu boot but cannot start windows.)

Ubuntu part:
ALT+F2➡mate-terminal
sudo su
cd /boot/efi/EFI/Microsoft/Boot
rm bootmgfw.efi
mv bootmgfw.efi.old bootmgfw.efi
grub-install
update-grub2

VOILÀ!

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