This is commonly referred to as the Blue Screen of Death (BSOD). It doesn’t mean your computer is permanently broken. A frequent cause of this problem is a device driver that doesn’t work with Windows 7.
If you recently connected or installed a new hardware device, disconnect or uninstall it. Then start the computer again. That’s your best bet. If you still get the Blue Screen of Death, you’ll likely have to boot to Safe Mode and disable the device through Device Manager. This is not the sort of thing the average user normally does. This is more the kind of thing that a professional would handle.
If the error persists, look for an error number on the Blue Screen of Death page. It will most likely start with the characters 0x. Jot that number down on a sheet of paper. Then, if you can get online through another computer, go to Microsoft’s sites (http://search.microsoft.com or http://search.microsoft.com) or your favorite online search site (such as Google) and search for that number. You might find a page that offers an exact solution to that problem.
If you can get online through another computer, you might also consider posting a question at the Windows Communities site. Be sure to include the error number in your post. You might find someone who has already experienced and solved that very problem.