White prohibitory sign on Startup Have the white prohibitory sign when trying to boot up MBP Retin 13" late 2012 running Yosemite. Have tried resetting PRAM (cmd + alt + p + r) and disk utility has been to no avail so far. Have tried repairing disk permissions, but get message saying that one of them has been modified and cannot be changed. 

Really need help, any help would be much appreciated
Connect in target mode to back up any critical files that are not backed up
Boot to recovery and run Disk Utility there to repair the drive.
If that fails, Boot to internet recovery and see about reinstalling the OS.
Boot to internet recovery and wipe the drive and install
Get hardware repair