My hard drive is blinking yellow light

Hard drives don't normally have lights on them. Do you mean the hard drive light on your case? This means that the motherboard is accessing the hard drive, and is nothing to worry about.

Tower indicator lights blinking yellow and green. CD drive blinking green. Fans running,
Monitor message, "no signal". All plug-ins are tight. Unit won't shut down unless power
Is cut off. No response to conventional shut down procedure. New HD? System board?
Thanks for listening...