Did I kill my yeast?

Yeast can survive in temperatures up to about 100ºF (38ºC). Anything higher will kill them. Ale yeast will go dormant if it gets into the lower 50s (11-12ºC), while lager yeast can go all the way into the upper 30s (2-3ºC) before going dormant. As long as you pitch the yeast into the wort after it has cooled to room temperature, you're fine.