Strange iMac start-up behaviour

Our power went out some time last night during a windstorm and came back on around 6 AM. My iMac was off and when I tries to start it I got no response from the on button.

I left it alone and roughly a half hour later I heard it chime. :clap:

Any idea what was causing the delay?

via ehMac.ca http://ift.tt/2mZwrUb