First things first, try restarting it in safe mode... Shut down your Mac and wait about 5 minutes. Then boot it up and at the same time hold the Shift key on your keyboard until the Apple logo shows up. If it does not, try this. Shut down your iMac and unplug it from its main power source. Then wait 5 minutes. After that, start up the iMac and instantly press and hold Command + Option + P + R all at the same time and keep holding it until you hear the start up sound 4 times and then release them. If this doesn't work... please reply to this message.
Hi, thanks for your response! I solve this by connect to other screen by vga and choose 2 screen as primary and works... The primary after 1 hour also turn on i dont know why but i see on main screen that have a part more dark than other so i suppose that the screen is damaged and after some time it will do again the same thing and wont turn on... The imac is a i5 - 27" from 2011 so i dont fix the screen because costs more than the computer. Thanks anyway!