Neptune's Harvest is pretty mild, so unless you poured it on undiluted (and maybe not even then!), I doubt it could do what you describe. Hydrangeas do get root rot if drainage isn't adequate, so if you've had a wet summer, that's a possibility. Most other diseases wouldn't kill a plant that way. Nor would most insect pests. If that's what it is, there may be little you can do at this point. But going forward, it will help to make sure the area isn't a low spot, or in the way of a downspout, or experiencing some other source of overwatering.