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Most likly a harsh winter or a late frost killed the blossoms. This article should help you: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm

Since both white plants are turning brown, that is a hint. I don't know where you live nor what the growing conditions are so this is hard to answer. The browning is typical of too much sun or too little water - this variety may require more water than the rest of your hydrangeas. I live in Texas and mine are being affected by the very hot, long-term heat. They are growing taller thus getting more sun - too much for our area. Where they once thrived as small plants, are struggling this summer.

This is very difficult to answer without a lot more information. Too much sun, not enough sun, too much water, not enough water, your current temperatures, etc. Take a couple of leaves to a good local garden center and get advice from their plant expert. Hydrangeas require a lot of water - try not to get water on the leaves especially if you live in a humid area.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm

It could be either black spot or sooty mold. Both are treatable and in most cases, not fatal to the plants. These articles will help:
(this is about black spot on roses, but it applies to all plants) https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm

Many hydrangeas bloom on "old wood." That means they are forming their blossoms for next year and by cutting off branches, you are cutting off next year's flowers. Some bloom on new wood (fresh branches produced in the Spring). I have found mine to be a combination of both. If they freeze to the ground and have all dead top growth, I get far fewer blossoms than in a mild winter where they keep their old wood (this past year). I don't prune except for dead material. There is a new hydrangea out called 'Pinky Winky' that is an exception to the pruning rule. I am replacing some of mine with it. Link for picture is http://www.provenwinners.com/plants/hydrangea/pinky-winky-hardy-hydrangea-hydrangea-paniculata