After the Windows 8 -> Windows 10 Upgrade the Debian testing UEFI Grub2 only showed it's rescue shell. On Debian, I use full disc encryption and have a seperate boot partition - here is how I fixed my Grub without using some LiveCD or Live USB:
On the Grub rescue shell, enter the following - in case you don't know which partition holds what, you can also try > ls (hd0,gpt7)/ [Enter] to get some content information. I knew that number 7 was my dedicated boot partition and number 8 the encrypted root file system.
grub rescue> ls # shows all partitions grub rescue> set prefix=(hd0,gpt7)/grub grub rescue> set root=(hd0,gpt8) grub rescue> insmod normal grub rescue> normal
Then I got the normal boot menu, including background picture again and could boot Debian linux completely.
Enter shell to make the changes permament:
# update-grub # grub-install
Partly source, which helped me finding the right way: https://www.linux.com/learn/tutorials/776643-how-to-rescue-a-non-booting-grub-2-on-linux/
(including the comment regarding dedicated boot partition)