Comment 4 for bug 613902

You may try passing reboot=bios on the kernel command line, please check if with it the reboot works. Some Dell machines require this and have quirks on kernel.

While on it, also please test these parameters to give to reboot= and report if they work or not: acpi triple pci

If it's a regression, please tell which kernel worked, and see if with newer upstream kernels it works again, you can follow directions on https://wiki.ubuntu.com/KernelMainlineBuilds and install a newer kernel on your system to check.