Looks like your hydrangeas broke dormancy too early and last Wednesday and Thursday's overnight temperature drops below freezing frosted the plants.
New foliage will grow in 2-4 weeks, depending on local weather. If any stems were killed, the shrub will develop new stems from the roots when it wants to. Flower buds that were opening or about to open may have been impacted.
The good news is that there are always back up flower buds lower down each stem and live buds may now produce blooms instead of the one that is at the tip end of each stem. The bad news is that the back up buds may have been frosted too. There is no way to tell which is which except to wait and see if you see broccoli heads. Healthy broccoli heads should be green. If they are partly or completely brown, the bloom was injured/killed.
Your average date of last frost in Buffalo is as late as the 3rd-4th weeks of May so be prepared for late frosts until then yearly. You can water deeply the night before a frost, mulch and cover with frost cloth/burlap/old sheets. I do not see a repeat event in the next 10 days forecast however.
Going forward, just water, mulch, winter protect. Fertilize after your average date of last frost (you could fertilize earlier but, any new growth triggered by the fertilizer could be frosted like in this year).
Leafless stems that remain leafless by the end of June or later can be pruned all the way down. If there are still live old wood stems by the time you get normally get blooms but these stems fail to bloom, consider if you might want to prune these live old wood stems to control their size.
If these cultivars are remontant ("rebloomers" in advertising jargon), the second flush of blooms should not be affected. For rebloomers, this second flush originates from new stems that the roots develop in spring 2024 and they immediately bloom (late summer/fall) once the new stem attains the proper height to trigger flower bud set and immediate blooming. Non-rebloomers will produce dormant flower buds in the summer for next spring 2025.
For videos on winter protection, consider doing a search in YouTube for 'hydrangea winter protection'.
