Mac Pro has grey screen with no entry sign .wont boot up
Mac book pro only has grey screen with no entry sign,won’t boot up
Mac Pro, iOS 11.4.1
Question marked as
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.
Boot into Recovery (Command R) and from the dropdown menu: Utilities> Disk Utility> run the First Aid on your Macintosh HD. If errors are found and repaired, run again until no errors reported.
Then reinstall the macOS—this can be done from the Recovery state you are currently.
Recovery http://support.apple.com/kb/HT4718
About the screens you see when your Mac starts up - Apple Support