Accidentally I pressed ctrl + Alt + F1 today in Ubuntu 16.0.4. It switches Ubuntu to text mod tty mode. There are 12 terminal in Linux 6 of them are text and can be switched by pressing Ctrl + Alt + F1 .. F2 uoto F6. If you are facing same issue you can get back to normal GUI by using Ctrl + Alt + F7 button together. If your computer have Function keys (Fn) you need to press it too. Means you may need to press ctrl+Alt+ Fn + F7