My 2 yr old iMac doesn't like being unplugged. Last night I had to un-plug it. 30 mins later i plugged it in. And pushed the on button. Every thing went well then the screen shows a picture and telling me to restart. So followed the instructions from the Mac web site. And pushed the internal on/off button. Doing that worked last time. But so far not this time. But why does my computer not like being un-plugged for long periods?