Macbook Pro 8,1 hanging on startup
There are 2 trouble shooting methods if the Safe mode fails.

But before you go in to them, try the PRAM reset.


  Turn on your Mac.
  
  Immediately press and hold the Option-Command-P-R keys. You must press
  this key combination before the gray screen appears.
  
  Continue holding the keys down until your Mac restarts, and you hear
  the startup sound for the second time.


If that did not help move to the start up in Verbose mode (hold v during start up).


  Verbose mode will load a terminal-like screen that logs the boot
  process in realtime to identify the source of any particular errors
  that may be preventing the computer from booting properly.


Once you have identified the source (where verbose mode stops processing), move on to the Single user mode([command]+[S]).


  This mode will boot the Apple computer into a terminal-only mode where
  bash commands may be entered to perform further troubleshooting from
  the command line.


WARRING...if you do not know what you doing in this mode do not do it.

Now... the next step will depend on the cause identified in the Verbose mode.