Completely new to this... Found a very basic mead recipe decided to give it a try, but I think I may have goofed. Making mead in a 1 gallon glass jug. Forgot to pick up distilled water, so I elected to boil the water I was going to use, thinking it would not only help with water impurities, but also would help with diluting the honey. Allowed the water to cool a bit but possibly not long enough.
When I added the yeast, I got very little reaction. Allowed it to stay in the sink overnight as the recipe says it would overflow the bottle… nothing of the sort occurred.
Is it possible that the water was still too hot and killed off the yeast? Is there a remedy, or should I simply start over..?
Any help is appreciated…

Without knowing what temperature you pitched (added) the yeast, it's impossible to confirm or rule out that suspicion. Could be numerous other things as well. What brand/strain of yeast? Dry or liquid? How much did you add? If it was dry, did you rehydrate first? How much honey & how much water - this can help us guesstimate the original gravity which could be a clue. I'm assuming you don't have much, if any, brewers test equipment yet.

Help us out with some more details & we can likely help you find the culprit. In the meantime, let it do its thing - there may be a fermentation lag. All hope may not be lost yet.

Temperature it sat at overnight, how long it was there... If the jug was cool enough that you could put your hand on it without discomfort, it was probably cool enough to not kill your yeast, but the other parameters Ozwald asked about would affect the health and viability of your yeast.