This has happened to me after a recent Security update from Apple. Went to restart, and Mac just hung for ages with pinwheel spinning. Eventually turned off at back of Mac Mini. Trying to reboot and it just hangs on grey screen with Apple logo and spinning grey dial.
I can boot into Windows using BootCamp not a problem, and I've tried entering Safe Mode on Mac by pressing Shift key. A grey bar slowly fills up about 1/3rd and disappears, leaving me with same grey boot screen with Apple logo & dial.
Anyone else experience this, or have any ideas how I can fix - please let me know.