Imac Flat Panel Won't Start Up

Hello, Our salon has a Imac Flat Panel G5. We have Been having problems with the computer starting up when pressing the button on the back. It would not do anything. I did unplug it and leave it unplugged for 5min. and it would start when holding the button for 10 seconds. A whistle sound would occur and then it would start. I have been leaving it on the last few days but this morning I went in it was dead. I have done everything and no nothing works. What can I do? John

Message was edited by: JMBHAIR1