When being forced into GRUB rescue mode, run ls and ls <one_partition> to check available partitions and find the one that we should boot from [ref].

Then set prefix & root[ref]:

set root=(hdX,Y)
set prefix=(hdX,Y)/boot/grub
insmod normal

To avoid doing this every time, it is recommened to use a live-USB/ISO to repair GRUB (usually only Desktop version can be in live-USB and easy to setup WIFI). (// it should be also ok to do this without USB if there is a nix system working on current HDD).
Then use boot-repair to repair the grub.

sudo add-apt-repository ppa:yannubuntu/boot-repair && \
sudo apt-get update && \
sudo apt-get install -y boot-repair && \

It will automatically repair it.