That's odd. That yeast should have plenty of room to ferment, and 2 months isn't that old. Do you have any more yeast you could pitch? I would make a simple starter of sugar, yeast nutrient, and water, and once it's going add a little of the mead to it (maybe 1/2 cup). Assuming that ferments, add a little more mead the next day for a few days, just to get the yeast acclimated to the alcohol environment, and then pitch the whole starter. That might get things going again.