Ever since yesterday, my PC never turns off when shutting down or suspending. The screen turns black but the fans are still on. I am using Ubuntu 18.04 LTS.
We done some troubleshooting one day with a other user :
Maybe something went out from this configuration after the upgrade.
I hope it can help you start troubleshooting
Setting
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force"
or
GRUB_CMDLINE_LINUX_DEFAULT="apm=power_off"
didn't work.
I tried to update the kernel with the mainline kernel installer but the Nvidia driver failed to build with those kernels.
Hi 
Sad to hear that.
What is your motherboard ?
My motherboard is Gigabyte H55M-S2
If nothing from ACPI settings helped, i can advise you to update your BIOS or try to replace the motherboard battery.