Is it one of these screens?
About the screens you see when your Mac starts up - Apple Support
maybe this:
Prohibitory symbol

When you see a circle with a slash symbol instead of the Apple logo, it means your Mac couldn't find a valid System Folder to start up from.
If you're using your Mac at a school or business, it might be trying to start from the wrong version of OS X. Contact your IT department for more help.
If this is your personal Mac, try reinstalling OS X by using OS X Recovery.
You can also call Apple since yo have 90 days of free telephone support
Complimentary Support (USA)