I have just run a software update on my MacBook Pro and now it will not re-boot. All I get is the grey start up screen and the spinning wheel - any ideas?
Turn off the computer. Turn on the computer while holding down Command, Option, P, and R. You will see the screen go black and the chime again, at that point release the keys and see if the system boots up
Look at this Apple support article:
See gray screen issue, run 1-8, you'll be booting again.
Step by Step to fix your Mac
Retrieving data ...