Hello. Try disconnecting the power cable... If this doesn't work, try having an outside vendor or the manufacturer fix it... Hope this helps...

Hello. Try looking at the PDF mentioned...

Hello. I also suggest trying to reboot the device... If this doesn't help, try booting into safe mode. If the system works there, it might be an OS problem...which needs repair... Hope this helps...

Hello. Just by looking at the LED meanings, I think the first 3 LEDs mean your iMac is working properly. The 4th LED is only on when your device is in "sleep mode". Hope this clears it up a little...