My iMac has an excess of five years on it. Recently, when I place it on sleep or stop working on it while answering a call etc., the screen freezes in black and it will not resume working again. In order to get it back on, I must either press the power button to turn it off or pull the plug. This is an intermittent event, that has occurred several times over the past six months. I am thinking that the monitor/screen is nearing its end life. Does anyone have any suggestions on what I should do about this problem?