I had a very similar problem.
I dual boot Windows 10 and Debian (BunsenLabs), and if it is relevant then I use the (U)EFI BIOS system. Here is how I solved the problem.
Yesterday, I opened Windows for first time since summer, and it needs to install updates and to reboot and at second reboot I get the GRUB rescue prompt.
error: unknown filesystem
(hd0) (hd0,gpt6) (hd0,gpt5) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)
In order to find a filesystem GRUB rescue could recognise I run through
ls (hd0,#) where the hashtag is a number between 1 and 6 (note: one can leave out the "gpt"-part and probably also the "msdos" part if you use the old partition table).
It turns out
gpt5 is my home partition and
gpt4 is my Debian root partition.
None of the other partitions had a filesystem that could be recognised (by GRUB rescue).
I now followed Carla Schroder's "How to rescue a non-booting GRUB 2 on Linux" guide. At the end of the guide she provides commands to boot from GRUB rescue. Notice when reading the guide, that at first she addresses the problem of booting from the normal GRUB prompt - that is not the rescue prompt, which is relevant to us, so read past that part, and you'll get to the rescue prompt.
Since my boot partition is
gpt4, I ran the following commands:
The last one made the font of the prompt change a bit, and then the command
which started the my normal GRUB 2 graphical boot menu. The guide didn't say anything about that, that would happen... but it worked splendid. I choose the Windows booter, and the update install continued. It rebooted another time, and sent me back into the GRUB rescue prompt, and I repeated the procedure, again choosing to boot Windows, and this time the update installation finished.
Now reboot manually (when the Windows update have been configured). Again I get the GRUB rescue prompt and I repeat, but this time in the graphical boot menu I choose my Linux (BunsenLabs), and when I have logged in I open a terminal and still following Carla Schroder's guide, I did
which finds my Linux image and the Windows boot manager, and then I did:
sudo grub-install /dev/sda
Finally I reboot again to check that everything works. I now directly get the graphical GRUB boot menu and both Linux and Windows can be chosen.
I hope this will be helpful, if anyone experiences a similar problem.