Reverting from Ctrl - Alt - F1

I tried to install an nvidia driver and there was an error saying I must not be in an X terminal when doing so. So I did a Ctrl+Alt+F1.

The resulting terminal did not allow the nvidia driver to be installed and showed the same error message. How do I revert back to the Unity GUI?

Ctrl+Alt+F1 to F6 are the virtual consoles provided by the getty/agetty programs. Ctrl+Alt+F7 is the console where your X server is running. The GUI (Gnome/KDE or any other) runs over X. So to get back into your GUI window manager: type:

Ctrl+Alt+F7

or

Alt+F7

or

Ctrl+Alt+F8

With 17.10 and newer, the login screen is on virtual terminal 1, and logged-in users' GUI sessions on VT2 and onwards. So, you may need to use Ctrl+Alt+F2 or Ctrl+Alt+F1 instead.

When you are in a virtual console just press Alt+RightArrow or Alt+LeftArrow to move to next/previous virtual console respectively. Console with X is usually seventh.

To revert to the GUI desktop you have to press:

Ctrl+Alt+F7

For completeness, I'll mention the chvt command. The virtual terminal offers a login prompt, and if you have logged in, you can switch to other VTs:

chvt 7

will probably get you back to the GUI, since it usually runs on VT7, as mentioned in the other answers. This is useful for scripting (for example, when taking a screenshot of LightDM).