When i’m done using my iMac I shut it off and when I come back to it the next day to turn it on it makes the chime I here the fans spinning but the display is blank. I try plugging it into an external monitor it says no signal. I turn it off unplug it wait 10 sec plug it same thing happens so I just keep hitting the power button and waiting for the chime over and over and finally the display pops up. (it does this about every time) Help please!