Stuck at the boot screen I'm running Yosemite. While booting up I get stuck at the screen with the following screen.

Is it a serious issue? If yes, how can I save my data?
The prohibitory means the system has found an installed OS but was unable to boot it. 

Your possibilities are : 


Boot into safe mode (Hold shift at boot and if it boots, fixing your permissions 
Backing you drive using another Mac and Target Disk Mode