Are you sure your thermometer is accurate? Overheating your yeast will definitely kill a lot of cells.
Also, how are you determining whether you are seeing signs of fermentation? If you're fermenting in a bucket and it isn't properly sealed, then air may be going out of the lid instead of the airlock. Also, mead does not produce a large head of krausen the way beer does.
If you're doing a staggered nutrient addition, then it should foam up considerably when you add the nutrients. That's a telltale sign that fermentation is going on. If nothing happens then I'd pitch another pack of yeast. Rehydrate that one in room temperature water or just sprinkle it in to avoid cooking that one (in case that was the problem).
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