iMac black-screened, wouldn't wake up; here's the error message
An occasional kernel panic is to be expected.  If you begin to see this message more and more often, consider basic troubleshooting as per Apple's instructions:


  To help diagnose recurring kernel panics, record the date and time it occurs, and any information that appears with the kernel panic message.
  
  Was the computer starting up, shutting down, or performing a particular task when the recurring kernel panic happened?
  Is the kernel panic intermittent, or does it happen every time you do a certain thing?
  Does it occur only when a certain external device is connected, or a device is connected to a certain port?
  Isolate hardware or software as the cause of the kernel panic
  
  Start the Mac from Recovery or its install media. Note: If a kernel panic still occurs, go to the "Hardware troubleshooting" section of this article.
  Open Disk Utility and use "Repair Disk" on Mac's internal hard drive (named Macintosh HD by default).
  
  Important: If Disk Utility is unable to repair the internal drive, you should bring the Mac to a Genius at an Apple Store, or an Apple Authorized Service Provider for service and support. Be sure to ask that, if the drive needs reformatting or replacing, they contact you about escalating your case to a special data recovery service. If you plan to visit an Apple Retail store, make a reservation at the Genius Bar using http://www.apple.com/retail/geniusbar/ (available in some countries only).
  
  Connect an external drive with at least 10 GB of free space. Note: Make sure the external drive does not cause kernel panics and is the only device on its USB, FireWire, or Thunderbolt port. Connecting the external drive and its cables to another Mac can help make sure the drive does not cause kernel panics. 
  Install OS X on the external drive.
  Start up from the external drive.
  Install all software updates until Software Update reports your software is up to date.
  Use the Apple applications on the external drive to surf the web, view QuickTime movies, email, print, scan, and/or other activities. Continue using the Mac for the amount of time it would usually take for the kernel panic to occur.
  If a panic occurs, go to the "Hardware troubleshooting" section of this article to further diagnose the issue.
  If a panic does not occur, go to the "Software troubleshooting" section of this article to further diagnose the issue.