Unplug all power connectors except for the main ATX and the ATX12V connectors to the motherboard. Also unplug all fans with the exception of the CPU fan (under no circumstances should you run the system with this disconnected). If your graphics card has power connector(s), leave it/them connected as well. Better still, if the motherboard has on-board graphics, temporarily remove the graphics card and use that instead.
Does the system now start up? Ignore the message about not being able to find the OS (which it won't, until the HDD is reconnected). If it does, reconnect each of the disconnected items one at a time and test again. Repeat until you find the culprit. Verify this by disconnecting it and testing again.