If I understand your description correctly, you're saying you have to hold down the power button and then, only after that, can you then boot up the computer again (by tapping the power button, I presume). The reason I'm guessing this is that holding down a computer's power button doesn't usually reboot it--instead, it usually just forces it to turn off. And that means it really was on before you pressed and held the button.
Assuming that's what's going on, this suggests your computer is actually not shutting down when you press Ctrl+Alt+F1, but instead that there is some problem displaying the console text (either a problem displaying all virtual console text, or a problem specific to whatever is running on
You can test for this and, if this is the case, fix the problem, by simply switching back to the virtual console that the GUI is running on, the same way you normally would: with Alt+F7 or Ctrl+Alt+F7.
If that works, you can try to work around your problem by using a different virtual console (for example, see if Ctrl+Alt+F2 works).
If you get get back to at least one other virtual console (even the GUI on
tty7)--and you haven't deliberately made changes to your system to keep
getty from running on
tty1, which I presume you have not--then this may be separated into two issues:
- Switching back to a working virtual console, as above.
- A bug interfering with the operation of, or the
getty instance on, one or more virtual consoles.
This answer (attempts to) address the first problem; the second isn't a question for Ask Ubuntu but instead should be a bug report for Launchpad. I'm not altogether sure what package it should be reported against; if you provide details of what happens when you try this, perhaps someone will know.
On the other hand, if your computer really has shut down or is otherwise truly nonresponsive after you switch to
tty1 with Ctrl+Alt+F1, then your entire situation is a bug (and the best you can do about it is probably report it on Launchpad). In this case, I can guess what package to report it against: the kernel,
linux (and you can run
ubuntu-bug linux to start the report).