When you get a GRUB rescue prompt, it means that the GRUB bootloader cannot locate its modules from the partition that contains them in
/boot/grub. The reason for this is that the code that fits in the drive MBR record is tiny, and so its main purpose is to locate and load the rest of the bootloader code. Once it's loaded its modules it can present the GRUB menu and handle the rest of the boot process. If it can't find the modules, it presents a very limited rescue prompt.
So, what you are seeing is totally normal if you repartitioned, thereby breaking the reference the bootloader had to the partition containing the boot modules.
Since you can boot manually, the simplest fix is to manually boot and reinstall the bootloader in the MBR so that it has the correct reference to your partition. The way to do that is:
sudo grub-install /dev/sda
This assumes you boot from
/dev/sda (most cases, but if not, replace it with the correct drive).
This solution is far simpler than using the
boot-repair, but if you are unsure this may be more suitable.