This is how I fixed it:
1) Boot from a Live CD
2) Reinstall grub as explained here:
https://wiki.ubuntu.com/Grub2#Recover%20Grub%202%20via%20LiveCD
Just in case the link above stops working, this is what I did:
# sudo mount /dev/sda4 /mnt
("/dev/sda4" is where my linux data partition -containing also "/boot"- can be
found. The other three previous ones are OSX stuff).
# sudo mount --bind /dev/ /mnt/dev
# sudo chroot /mnt
# upgrade-grub
# grub-install /dev/sda
(yes... this is "/dev/sda", without the "4")
Once it boots again, the "update manager" does not find any new grub
package... so I guess this won't happen again (after all the new grub
was installed, we just re-created its config file)
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After 9.10 grub update can not boot into Wubi install
https://bugs.launchpad.net/bugs/477104
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