Use Ctrl+Alt+Fn to switch back to a virtual console. Ctrl+Alt+F1 would return you to the first virtual console.
Once you are back to a text console, you can then use Alt+Fn as normal to move between them.
To return to the X session, you must switch to the virtual console running X. If you invoked X from the command line, (e.g., using startx) then the X session will attach to the next unused virtual console, not the text console from which it was invoked. If you have eight active virtual terminals then X will be running on the ninth, and you would use Alt+F9 to return.