Keyboard and mouse not recognized

You boot your computer, as usual.
You get to you login manager, as usual.
You cannot enter your ID/password because your keyboard is not recognized, DAMNIT!

You're screwed, you need a live distribution to boot on and then access the log file of the X server.

Inside the '/var/log/Xorg.0.log', you may see lines beginning with '(EE)', meaning there is an error.
In particular, if you're seeing that the drivers for 'kbd', 'mouse' and 'libinput' are missing, you'll need to install them.

So first chroot into the system and then apt them.

# cp /etc/resolv.conf /mnt/guest/etc/resolv.conf # For the network
# mount --bind /dev/ /mnt/guest/dev/
# mount --bond /sys/ /mnt/guest/sys/
# mount --bond /proc/ /mnt/guest/proc/
# chroot /mnt/guest/

# apt install xserver-xorg-input-mouse xserver-xorg-input-kbd xserver-xorg-input-libinput
# apt install xserver-xorg-input-synaptics #This one is for the touchpad, not essential but add features.


And after that, you're good to go.