MacBook Pro showing prohibitory sign on start up
You may need to erase the entire drive. Once you've booted to recovery, use disk utility to repartition your internal drive. 


https://support.apple.com/kb/PH20571
https://support.apple.com/kb/PH20567


Bear in mind that repartitioning the drive will erase everything.  If you don't have a back up of your data and every app (iLife/iWork are not part of the OS) that you want to install, repairing the disc might be a better choice.