Mac startup prohibitory sign is simply a warning which indicates that there is a problem in your system. It is one of the most critical boot time error on Mac. In such situation is normal boot is interrupted by the universal prohibitory sign and startup get struck with a gray Apple logo. In such situation users are completely unable to log on their Mac system. There are certain reason which can cause emergence of this very irritating and hazardous error Mac startup prohibitory sign in your system such as damaged or missing crucial boot element, improper installation of OS X bootable, problem in hardware or software settings etc.
However, there is no need to be panic at all. You can fix this issue quite easily on your Mac system. Under such calamitous circumstances you will need to reinstall the Operating System in order to get rid of this frustrating as well as devastating Mac startup prohibitory sign on your system. If you are concern about your data then just calm down because you can easily get back all your files even after reinstalling OS with the help of Mac Data Recovery Software. Hence it is recommended to reinstall your Mac OS X now and get rid of Mac startup prohibitory sign easily.