What does ø icon mean when restarting MacBook Pro?

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.