How to restore a deleted Ubuntu kernel / python

Rule NR.1 always keep at least 2 working kernels installed on your system!
For more information on the Ubuntu server administration, I would recommend taking the Practical Ubuntu Linux Server for beginners course.

This is a very painful exercise, but if you are careful during the steps it will be beneficial.
First, boot into liveCD or liveUSB and ensure you have networking.
mount the partitions:
here /dev/sda2 is the partition of my hard drive, where the kernel is broken. 

you can check yours by typing: lsblk
sudo mount /dev/sda2 /mnt 
sudo mount --bind /run /mnt/run
sudo mount /udev /mnt/udev
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

enter the hard drive environment:
sudo chroot /mnt

mount -t devpts none /dev/pts
from now on you are working back as if you have booted from your hard drive.

Then wait to finish: apt update && apt dist-upgrade

Note:

if in this step you experience problems such as:

dpkg: error processing archive /var/cache/apt/archives/gcc-10-base_10.2.0-13ubuntu1_i386.deb (--unpack):
 trying to overwrite shared '/usr/share/doc/gcc-10-base/changelog.Debian.gz', which is different from other instances of package gcc-10-base:i386

just install the package with --force-overwrite from the cache:

sudo dpkg -i --force-overwrite /var/cache/apt/archives/gcc-10-base_10.2.0-13ubuntu1_i386.deb
and re-run: sudo apt dist-upgrade to complete the installation of the packages

(repeat this for every error until: apt dist-upgrade completes, it may take you some time while installing all the dependent packages resulting in errors)

If you are just missing the old system i.e. having deleted python libraries, do sudo apt install ubuntu-desktop , exit the chrooted shell, and reboot the computer.

Now to the kernel restoration process:

We will skip the grub loader update which may prevent some of the next installations or removals of packages:

sudo mv /etc/kernel/postrm.d/zz-update-grub /etc/kernel/postrm.d/zz-update-grub.bad

Check what version of the kernel are you running with uname -r
then type dpkg -l | grep linux-image
and find at least one kernel starting with ii -> which means it is correctly installed on your system and can be used to boot from later.
- if there is none kernel installed, go ahead and try to install some:
apt install linux-headers-generic
apt install linux-image-generic
- if you experience problems while installing this kernel, install a custom kernel of your preference. The trick is to have the kernel with ii in front!
- if you have problematic kernels(those marked with rH (half-installed) or other flags such as rc (removed)) you can remove them first with apt purge kernel_version...
restore the grub loader updates:

sudo mv /etc/kernel/postrm.d/zz-update-grub.bad /etc/kernel/postrm.d/zz-update-grub

lastly, get your new kernel settings inside of the grub loader:
update-grub

Then exit the chrooted shell and reboot.

One last thing, since we are using USB/LiveCD update-grub could detect also the OS inside those devices, and bloat the whole boot menu.
So after rebooting just re-create the grub configuration with: sudo grub-mkconfig -o /boot/grub/grub.cfg

Congratulations and happy learning!