I'm not an Ubuntu expert, I stick mainly with CentOS or Fedora. But can try to help.
What version of Ubuntu is it?
So when you boot without the external monitor, are you seeing the grub menu?
Assuming you are seeing the grub screen can you tell if it is grub or grub2? It should say on the screen. My guess if a new Ubuntu its grub2.
Now this isn't a fix, but its where I would first start myself.
Keep the external monitor unplugged.
When booting, press a key to stop grub from automatically bringing up the OS. Hopefully you haven't done something like set the timeout to 0. :>
Now here's where my suggestion may not exactly work, as it will differ between grub classic and grub2. Add " single " to the kernel line, make sure you pick your default kernel(normally would be the newest/highest version)
This should get you into single user mode. In most all Linux distros I have played with, not matter what your monitor setup is, they play nicely with them in single user mode.
Hopefully here you got a login prompt and you can log in. Well I'll stop here, as I don't know if you get this far. But at this point you can start playing around with things.