Since you haven't ever opened it, your best bet right now would be to do so, and clean out any gunk/dust/debris out of the cooling system, and heatsink. Then, you may want to replace the thermal paste under the heatsink. Once you're done that, then you can try it out again. If it still doesn't work, then you should look into swapping the hard drive, or replacing the capacitors if they are bulging, or leaking.