How to prevent computer from automatically sleeping and/or hibernating?

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http://askubuntu.com – I'm running Ubuntu 12.04, and my laptop* won't wake from sleep/suspend/hibernate. (Is sleep the same thing as suspend?) I'm not even sure which of these things it's doing. When I am done working for the day, I lock my screen (Control-Alt-L). When I come back the next day, the screen is in power saving mode, and no amount of typing or clicking (on the usb keyboard/mouse or the builtin keyboard/trackpad) nor tapping the power button will bring it back to life. The only way I can get my machine to work is to hold down the power button until it shuts off, then press the power button again to turn (HowTos)