I have a double boot Windows 10/Linux Debian and updating Windows to the october 2018 update (1809) made grub unable to start itself at the 3rd reboot (I think) of the update. It went straight to grub rescue with unknown filesystem type.

To solve this problem, you have to manually direct grub to the right startup file to boot normally. First, do a ls to take a look at the partitions available, you'll get a list of the form: (hd0) (hd0,gpt7) (hd0,gpt6) (hd0,gpt5)... If you know where your grub configuration resides, you'll use the correct (hdX,gptY) directly else you'll have to try them in turn until the following commands do not throw an error when doing insmod. In my case it was (hd0,gpt6).

set root=(hd0,gpt6)
set prefix=(hd0,gpt6)/boot/grub
insmod normal
normal

Normally now grub should start as usual. Boot on linux and repair the grub installation. For me under Debian it was only a matter of (sda being the disk from which the computer boots in my case):

sudo install-grub /dev/sda
sudo update-grub

And then on reboot it went correctly to the grub menu. Hope this helps, as the problem seems to happen regularly with windows big updates :-|