Unfortunately we can't see enough detail in the photos to be certain about what is going on and what species of evergreen these are. (Our best guess is leyland cypress, based on what little foliage we can see.)

Several possibilities can cause leaf loss on conifers (needled evergreens like leylands, pine, spruce, juniper, yew, etc.).

• Bagworms eat foliage and can strip parts of a plant in one season if numerous enough. We don't see the characteristic bags of these caterpillars in the pictures, but they might be too cloudy or blurry to be certain. Information on the linked web page will tell you what to look for.
• Fungal diseases like Seiridium and Botryosphaeria canker can kill branches, and tend to infect plants that are first stressed by high heat or drought. (Many areas of Maryland experienced a season-long drought in 2023.) Damage this extensive is likely not caused by only this issue by itself, though, since these diseases often don't kill this much growth in one year alone.
• Over-pruning will remove foliage that older plants cannot replace. Most conifers, including Leyland Cypress, have a limited ability to regrow foliage that has been removed by either pruners or deer browsing. (Deer can eat Leyland foliage.) Leyland Cypress are large-growing plants, easily reaching at least 25 to 35 feet high or more and 10 or more feet wide, so pruning them much smaller than this will eventually result in bare areas that never fill back in.
• Recurring stress from over- or under-watering/drought can cause leaf loss on the interior of the plant, though some degree of bare interior features are perfectly normal for Leylands and many conifers.

In this case, our intuition is that the plant was pruned too heavily over the years. Regardless of the cause, the bare branches will not be able to regrow foliage, so they will never become green and leafy again. Given the plant's appearance, then, it makes sense to just replace them. If pruning was done to make them fit into a limited space, then select something that matures smaller so pruning will not be needed. If pruning was done in part to give the plants a very manicured-looking shape, then consider using another kind of plant (a broadleaf evergreen, like holly, instead of a conifer) that will tolerate that kind of repeated pruning. Conditions like how much direct summer sun they receive how well-drained (or wet or dry) the soil is, and if deer are an issue, will all influence what substitute plant types are appropriate so they can be expected to thrive.

Fertilizer is only beneficial to plants in good health; it can be ineffective (at best) or harmful (at worst) when used on plants suffering from a pest or disease issue, and it cannot force a plant to regrow foliage on bare branches if that plant normally does not regenerate that way. Fortunately, most plants have plenty of the nutrients they need in average garden soil, and routine or even occasional fertilization is not needed.

Miri