Sorry to hear of your Mac issues, KOB. Something similar to that happened to me a couple of years ago. Believe me, I know that sinking feeling!
Does your MBP have an optical drive, or is it one of the new ones (Retina MBPs have no optical drive)? You do have your data backed up, do you?
You may want to try and boot up from a system CD (if the answer is yes) or from the
Recovery Mode (for OS X Lion or later). If the computer boots up into the desktop from the CD or Recovery partition, then you likely have a glitch in the update process (i.e. a corrupted download).
In that case you'll want to reinstall the system from either the CD or Recovery Partition and get to to the desktop, or restore from a Time Machine backup that predates the update. Then you can try re-downloading and reinstalling the update again.
If the computer won't boot up from CD or Recovery partition, then you may have a hardware issue. That requires a visit to the nearest Apple Store or Apple Authorized Service Technician.
I hope this helps! Do let me know how it goes for you.
This post has been edited by haute ecole rider: Apr 14 2014, 04:44 AM