It could be many a things.
It could, first, be the PSU. What make/model/wattage do you have?
Does the system start up (POST) fine, without the HDD connected? It'll just say there is no boot disk. I would, at first, back up everything, just in case your HDD is indeed failing. Get diagnostics for your disc. Get this: WD Support / Downloads / SATA & SAS / WD Caviar Green / GP
Might also want to post a screnshot of HWMonitor. So we can see your temperatures and PSU voltages.