If someone tells you to "Get your head out of your ass," it means that you have to start paying more attention to what's going on around you. It could also mean that you have to stop being so consumed with yourself, and your own well being, stop being so conceited.
John: I am so good at painting, I must be like Michelangelo!
Getting your thumb out of your ass means to stop wasting time doing nothing when you've got shit to do.
"Ugh, I hate English. It's so boring!"
"Yeah, well this is a Group Project, so you're not the only one one whose neck is on the line. So get your thumb out of your ass and help, you lazy fuck!"