I suggest you start by trying to follow the steps in this guide: DebuggingACPI.
The description at the top of that page says it is part of Debugging Central, "pages with debugging details for a variety of Ubuntu packages" on the Ubuntu Community Wiki.
You should also try looking at the entries in /var/log/syslog
to see if anything there will help isolate the problem.
Make sure the portion of the log you look at is from a failed boot. That is, the log entries should be from a boot where neither acpi=off
nor nolapic
were used. The log timestamps should allow you to determine which boot a log file entry is from.
If you find it easier to use a GUI application, Ubuntu has a Log File Viewer
which you can use. It can be found by searching for it using Dash
. 
The steps below are (pretty much) copied from the DebuggingACPI page and are what I suggest you do to try to isolate the problem before filing a bug report. (I believe the procedures for reporting a bug are also on the DebuggingACPI page)
If acpi=off
allows the system to boot, try to isolate the ACPI issue with the following boot parameters. Remove acpi=off
and boot with only one of the options below.
If you are lucky, you might find a combination of kernel parameters which allows you to boot and use all the cores of your CPU.
Note: If you need an explanation of to how to do a "one time" change of the kernel boot parameters/options, try this answer to the question "How do I add a kernel boot parameter?".
- Try booting with
acpi=ht
This disables all of ACPI except just enough to enable Hyper Threading.
If acpi=off
works and acpi=ht
fails, then the issue is in the ACPI
table parsing code itself, or perhaps the SMP code.
- Try booting with
pci=noacpi
This disables ACPI for IRQ routing and PCI scanning.
- Try booting with
acpi=noirq
This disables ACPI for IRQ routing.
- Try booting with
pnpacpi=off
This disables the ACPI component of the Linux Plug and Play code.
- Try booting with
noapic
Disables the IO-APIC for IRQ routing or PCI scanning.
- Try booting with
nolapic
Disables the local APIC.
acpi=off
&nolapic
? What happens if you do not use those kernel options? Having only one CPU core is apparently a result of disabling ACPI. See this question: Is SMP running on my computer?