Initialization seemed slower than usual and when the Apple logo appeared I got this long, oval shaped progress bar below the spinning wheel. I had never seen such a thing on my Mac in over 5 years... And the progress bar on my Mac was seemingly not making any progress, which was worrisome.
This computer had been a total workhorse for me, putting in 12 hour days, easily, and more or less handling everything I've thrown at it for over five years. But now it was not working at all. The progress bar would never advance, the spinning gear kept spinning and then the iMac would shut down automatically, and not reboot.