I agree with Darren. However, the heat that my laptop generates seems to have caused my battery to deteriorate quicker than it should have which resulted in my having to buy a new battery last week. I also bought a new Belkin Cooling Pad at the same time for less than £12 from Amazon (incl delivery!) and the laptops temperatures have dropped quite dramatically. I don't use them now but I've noticed my previous two Acer laptops tended to get hotter than my other laptops and my last Acer actually fried from a hardware fault. Maybe this is relevant to the OP.
Anyway, going back to OP. The diagnostic tool could be just about anything. I would recommend looking at the Event Log Viewer first as this might give you some clues as to why the computer shut down. More info here.