If you just upgraded from Ubuntu 16.04 to 18.04 and the system won’t boot properly, the reason could be a failed GRUB installation as it is mentioned here.
To solve this issue after the upgrade you have to boot into the rescue system as a first step. As soon as you have access to the rescue system execute the following commands:
mount /dev/vda2 /mnt grub-install --boot-directory=/mnt/boot /dev/vda
Should you still have troubles booting the system after following the above mentioned steps, please contact us by mail or phone.