I have Ubuntu 12.04 installed on my Acer C7 chromebook. Once I installed it, I never switched back to Chrome OS. But yesterday when I got back home, I found that my laptop LED was on, and the Chrome OS was running. How can I go back to my Ubuntu 12.04? I usually had a screen that has a chrome logo on it every time I turn on my laptop, where I always pressed Ctrl+D to run Ubuntu, but this screen no longer appears when I boot.