The GRUB boot loader can be entered by holding SHIFT or ESC during system startup. Once the GRUB loader has started, select an alternative kernel than the default / most recent kernel installed. This should allow the system to boot.
In general the recovery procedure is the same. Space must be freed in the /boot partition by removing older / broken / obsolete kernel packages but the procedures may vary depending on current deployment.
Once the system has logged in via the alternative kernel, capture and provide the output of the following commands, and open a GTAC support case:
ls -al /boot
ls -al /boot/grub
apt-get install (select NO if/when prompted)