Has it ever worked, or did it just start doing this? Have you changed the hardware at all? Try removing all peripherals, make sure CD drive is empty, just in case boot process is hanging on some piece of hardware. Or try the Ubuntu forum where people might be more familiar with features of Ubuntu boot sequence.
As an emergency measure you can try to boot off a live CD. Depending on your skill level you can then either check and fix the Ubuntu boot configuration, or backup your data and try a re-install.