Eu tive o mesmo problema. Na minha opinião, o problema está relacionado com a incompatibilidade entre o kernel 4.15 e o driver da Nvidia (340, no meu caso) .Em passos que tomei para resolver o problema:
At the boot I pressed Esc and went to the grub options.
I chose the 4.13 kernel and the system booted up.
I installed grub-customizer and changed the kernel position at boot, to avoid do the same each time... But this process is not so complicated because we had already the previous 4.13 kernel installed, because this was not a 18.04 fresh install.
Opcional:
You may enter the following command in a terminal: sudo apt install haveged,
como descrito neste bug aqui; link