Ubuntu – Changing current directory to shared filesytem using linux terminal

In theory, a Windows update should not have affected GRUB's ability to load its configuration file, which is what appears to have happened. I have two ideas about what has happened, and fixing one of them might correct the problem:

• Changed partition table -- If Windows modified the partition table, then that might have invalidated some partition references in GRUB. If so, you'll need to re-install GRUB, or at least adjust its configuration file, as described shortly.
• (Re-)enablement of Fast Startup and/or Hibernate -- These features can cause filesystem corruption, which might prevent GRUB from reading its first-stage grub.cfg file on the ESP. If so, disabling these features might fix the problem. I therefore recommend you start by doing precisely this. See this page for information on disabling Fast Startup and this one for information on disabling Hibernate.

I recommend you look into Fast Startup and Hibernate first. If disabling those features doesn't help, then you'll have to look into more radical solutions. These include, in order of radicalness:

• Update your GRUB configuration -- Perhaps updating your GRUB configuration will help. Since you're able to boot, you can do this by typing sudo update-grub in a Terminal window in Ubuntu.
• Re-install GRUB -- Typing sudo grub-install will re-install the main GRUB EFI binary, which might fix the problem if it's the source of the problem.
• Use Boot Repair -- The Boot Repair tool is an automated tool that helps with re-installing GRUB. It's unlikely to do any better than re-installing GRUB with grub-install, but if the problem lies in a narrow range of issues beyond what grub-install can handle, Boot Repair might do the trick.
• Switch to another boot loader -- Other boot loaders exist, and some of them are more reliable than GRUB in some cases. My own rEFInd boot manager, for instance, is less reliant on configuration files, and so is less likely to be thrown by problems related to them. You can test rEFInd on a CD-R or USB flash drive (images for both are available on its downloads page) and install the PPA or Debian package (or from the Ubuntu repository, if you're running Ubuntu 17.04) if you like it.

I recommend starting with updating your GRUB configuration file and moving to more radical solutions later, on the off chance that something drastic (Boot Repair, in particular) could cause serious problems. rEFInd run from a USB flash drive or CD-R is unlikely to create new problems because it changes nothing on your disk when run in this way; only if you install it on your hard disk will it make any permanent changes.