What can I do to unbrick my MacBook Pro after a software update?
There are several other options your can try. First try rebooting/powering up and after your hear the chime press and hold the option/alt key. This will take you to the UEFI boot loader where you will be presented with all the boot options OS X recognizes. If you can, select the hard drive that contains your OS X installation.

Should the boot loader not launch or launch and then present you with no options reboot and then try pressing and holding command + r after the chime. This will either boot off of your recovery partition or attempt to boot your system from internet recovery. If it's the latter it will take some time so be patient. 

Once you are loaded into recovery mode launch disk utility from the tools dropdown. Select the first entry for your hard drive in the side bar and then click "Repair Disk".



This will attempt to repair any errors with your partition map. Reboot and then hopefully all will now be functioning correctly.