OSX Yosemite Prohibitory Sign on Startup I performed hard restart on laptop after it froze during a restart. 

When loading again, the apple logo displayed with a loader then displayed a prohibitory sign. After some time reading and many failed attempts, I have been unable to restart mac successfully - 

I am unable to boot into safe mode or single user mode to run disk utility as suggested by other apple documents. I even tried a PRAM reset. Is there anything else I can try?

UPDATE: holding cmd+r on restart allowed me to properly enter disk utility.
Wondering what you meant by PRAM reset due to the fact that technically, on the Intel models, it's now called NVRAM even though you still have to use the P and R keys. If you have ethernet, you can hold down command-option-r at startup to boot to directly from Apple and run First Aid in Disk Utility. If you have a battery that can't be readily removed, you might want to reset system management (SMC) by ensuring that the Mac is turned off, then disconnecting everything except power, then pressing and releasing together shift-control-option and power on/off (eject) buttons. Then do your NVRAM reset again. That can help about half the time. (NVRAM reset is starting then immediately pressing and holding command-option-p-r until the startup chime plays a few times, at least three.)