Certified GKH Gardening Expert
It is difficult to say for sure. There are a few things that can cause this. Holly scorch is the most common. This happens when the weather changes too quickly. The plants should recover from this on their own. Usually brown leaves signify that they need more water. Give them a good dose of fertilizer to help them get back to full health. Using root stimulator solution on the plants once a month for at least six months may help too, provided that the issue is not root rot. Another possibility could be holly leaf spot. Here is an article that may help: https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-problems-holly-leaf-spot-or-holly-tar-spot.htm
If the bark is still green, the plants are still alive, and even if they lose all or the majority of the leaves, they should eventually leaf back out.