Why a system crash after latest kernel upgrade to 4.15.0-24?

System: Custom, Intel(R) DZ68DB, Nvidia(R) GeForce 8400GS, 16GB ram, Intel(R) Core(TM) i3-2100 CPU @ 3.10GHz

OS: Ubuntu 16.04.2

Problem:

• Latest upgrades included kernel 4.15.0-24.

• Performed upgrade and upon reboot, the system boots, then screen goes black, bottom half of screen shows green horizontal lines for a second, goes black, and just keeps cycling that way.

• Tried shutdown/restart and the same thing occurs.

• Rebooted system and chose previous kernel 4.13.0-43 and the system booted up and is running fine.

So...has anyone else had the same problem? I would like to upgrade to 18.04 but am now somewhat spooked.

I had the same problem. In my considered opinion, the issue is related to the incompatibility between the 4.15 kernel and the Nvidia driver (340, in my case).

Steps I took to solve the problem:

1. At the boot I pressed Esc and went to the grub options.
2. I chose the 4.13 kernel and the system booted up.
3. 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.

Optional:

4. You may enter the following command in a terminal:

    sudo apt install haveged, 
   

as described in this bug here