As Dave explained, the reason is almost always Hybrid Graphics, especially if it's AMD Enduro. Just to make sure we have everything covered, you might want to install your GPU driver and make sure they work.
Most users would use the
Additional Drivers in Settings, but given that this is all proprietary , I like to go to the source.
Note that the drivers from "Additional Drivers" are neither the latest drivers, nor enhanced for performance on linux for many reasons, and while they might appear to work, that is never the case. Long story short, Just use the ones you download directly from the site.
AMD : http://support.amd.com/us/Pages/AMDSupportHub.aspx
There's a box "Top Support Articles" you'll find the latest drivers for linux there, you'll want the drivers there and not any place else, trust me, I have a mobility 7670 myself and these are the only drivers that worked for me.
Nvidia : http://www.geforce.com/drivers
After you download the file, refer to the top answer on
/questions/18747/ the part that explains how to install drivers from .run files.
Easiest way to make sure the driver is working after you reboot, search "About" in the lens, open it and choose Graphics, see if the driver is detected, if it says "Unknown" then the installation will have failed.
After you have confirmed that the driver is working, if the problem still persists, then it is most likely with the kernel not containing drivers for some of your pc components, especially if you have an ivy bridge, as Intel has dedicated separate on-board power management in ivy bridge.
If that is the problem it will be difficult to fix, there's an easy fix but not really the best.
See, in Ubuntu power management is not very efficient, it always works without error just not as efficient as it could get, to spare you the details, all you should know is that Ubuntu runs things quite differently from other distros and even Windows just to ensure user's peace of mind, I'm a Slackware user myself and in Slackware power management is given directly to the login shell (almost like everything else) and not running in the kernel as a part of an init block, which could create problems, and for that Ubuntu doesn't employ these methods.
Anyways, there's an easy fix for that, to install XFCE desktop. It has its dedicated power manager and it'll run things for you, overriding the power management in the Kernel since the login environment takes precedence.
Easiest way is just install XFCE and all other bound packages from Synaptics package manager, it's not the best way, but given that you're still new to Ubuntu, it'll save you a lot of effort.
You don't have to use XFCE or anything, just the power management package being installed will get things on its own (the scripts are automatically loaded with init).
This works for me on my DELL Inspiron.
The other way to fix it would be to replace the scripts in Ubuntu in
/dev with those that work roght, and load them into the kernel for power management to work, but then you'd have to compile and load and the process is quite complicated.
Comment if you have questions, and when you fix it, tell us what exactly did it for you.