Use magic SysRq key combinations. Try pressing Alt+SysRq+K to kill the boot sequence. This might let the boot sequence progress until you get a login prompt, but it probably won't. If it doesn't, press the magic more-or-less-safe reboot combination: Alt+SysRq+S (sync), Alt+SysRq+U (unmount), Alt+SysRq+B (reBoot).
Once your computer reboots, at the boot prompt, edit the kernel command line (the one that contains
/vmlinuz or something similar) to add the parameter
init=/bin/sh and boot. The system will boot straight into a shell without starting the problematic ACPI script. Remount the root filesystem read-write and disable the problematic script. Then reboot, and this time boot normally.
mount -o remount,rw /