2156 Views 1 Reply Latest reply: Dec 12, 2009 9:32 PM by Fox.Knight
It could be the power supply, if disconnecting and reconnecting the power cord makes it work temporarily. Disconnecting the power cord is resetting the hardware, so you can attempt another startup.
If you have not already, be sure to try resetting PRAM
and then resetting SMU (power management)
Be sure to follow the procedure precisely. When you reconnect the power cord, if you have been using a crowded or old power strip (surge suppressor), try connecting it directly to a wall outlet by itself, at least initially. When you power it up, have everything disconnected except for the power cord. If it starts up properly, connect only your keyboard and mouse. Use it that way for a while, and note if this problem recurs. If all is well, reconnect your other peripheral (USB/FireWire) devices, one-by-one if possible. If the problem returns at some point, note what device was added last. It may be faulty or conflicting with the iMac. USB and FireWire devices can interfere with sleep, wake, shutdown, and startup.
If the problem recurs, even after the SMU reset and attempting to start up with nothing but the power cord connected, it is very likely to be a hardware issue that require some type of repair.