It was actually two issues. When I shut down the computer I would get this error message: “reboot: system halted acquire the ‘org.freedesktop.ModemManager1’ service name nm-dispatcher.action: caught signal 15, shutting down… ModemManager.” And then it would just stop. It wouldn’t shut down. I had to physically push the off button.
Here is my solution: Since I am using a desktop, I just deleted the ModemManager. ModemManager is for mobile support, you don’t need it on the desktop. After this I got a shorter message, “reboot: system halted 15, shutting down.” But it would still hang.
My next step was to add acpi=noirq to the kernel arguments:
sudo vi /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT=”acpi=noirq quiet splash”
Be careful editing this file. The second link below tells you how to save and close the file.
Finally, make sure acpi is enabled in your bios. Hopefully that should fix it. It worked for me.