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There are a few things that can cause this, with holly scorch being the most common. This usually happens when the weather changes too quickly. The plant will normally recover from this on its own. Are you seeing any other symptoms? That will help us pinpoint it better.

This article outlines possible reasons for leaf drop: https://www.gardeningknowhow.com/garden-how-to/info/plant-dropping-leaves.htm
You've had a very hard winter, and it's quite possible your holly is suffering from this. Wait until spring is well underway, to see if there are signs of new growth. You can then cut dead growth back to where it is starting new leaves, or showing green tissue under the bark.

You can prune now, but if you do, you will likely not get berries this year. It doesn't hurt the plant, but as most people like to get the berries, they avoid pruning at this time.

This article will help with that: https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-cuttings.htm

If you have had a lot of rain and the drainage in the area is not good, it can cause problems with leaves. Is the ground swampy? If the ground stays wet, the roots can't 'breathe' and falling leaves or yellowing would be a sign of this. Too much water causes iron chlorosis. This article will help you fix the problem: https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-yellow-leaves.htm

It sounds like sooty mold, which is a symptom of several kinds of pests that can attack hollies. I recommend treating the plants with neem oil. This will kill any pests on the plant and treat the sooty mold as well. This article will help: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm