It might be simplest to start over with a new plant, for several reasons. What caused the original dieback is hard to determine at this point and may have begun in a prior year, but Viburnum is one of many shrubs vulnerable to the opportunistic pathogen Botryosphaeria canker, so that is a prime suspect. Drought stress is a common precursor to infection since this fungus generally isn't a problem on healthy, vigorous plants. (Even though the linked page focuses on Rhododendron, the fungus affects many plants.) An old pruning cut in the second image appears to have been made too close to the adjoining branch and wood decay set in before it could seal over, which may also be one contributing factor. We also see girdling roots around the base of the "trunk," though can't tell if they are advanced enough to have "strangled" any of the major stems.

For dieback of this type, leaving branch stubs like this is not recommended; it's always best to prune dead/dying limbs back to just beyond a junction with another branch. Given the congested branching in this situation, though, that will be difficult, and instead removing all the old wood that is dying back anyway would be best (if you keep the plant). That would be easiest to achieve by using a folding pruning saw, since it has a curved blade that helps it fit into tight spaces. You'd essentially be cutting-down all old growth, keeping the four newer and apparently still-healthy stems to let them rejuvenate the plant over time. If too lanky, they can be shortened a bit next spring, after blooms have faded, or during winter, if you don't mind losing some of next year's flowers.

The plant appears to be a Doublefile Viburnum (Viburnum plicatum tomentosum), the species of which has behaved a bit invasively in Maryland, seeding into natural areas in several counties thus far. Given that, it might be better to replace it with a different species, given the opportunity, if you are going to remove it anyway. There is a similar-looking native Viburnum, Hobblebush (Viburnum lantanoides), but it is much harder to find at nurseries. If you don't need a replacement to look too much like the old plant, then there are several other native Viburnum species in Maryland, though deer do unfortunately tend to browse on them, if deer visit the yard.

Miri

Hello Paula,

Thank you for the additional photos. The second image (ending in -37) is showing a classic sign of Ambrosia Beetle infestation...the "frass toothpicks" as we tend to call them. (Toothpick for the size and overall shape, and frass is a term used for insect poop, which in this case is mixed-in with mostly sawdust created by the wood-boring beetle.) The linked page by Ohio State presents a nice explanation of these and related beetles. Damage from these kinds of borers is untreatable, and often the insects target trees and shrubs that are under stress, such as from drought or over-watering. By the time the outward indications of an infestation manifest, the beetles have been in the wood for some time.

We can't tell why the lumpy growths are on the bark, but that too is not an encouraging sign of health, so given that and the peeling bark off another dead/dying limb in the photos, we still recommend that all old wood be cut down or the shrub replaced entirely. Prior branch removal that was cut too close to the junction point likely contributed to the holes in the wood, since the bark was not able to close-over the wound. Callus tissue exists in a ring around where a branch joins a trunk (or another branch), and if this specialized tissue is wounded when a branch is cut off, wound closure cannot happen and wood decay is very likely. Nothing can remedy wood decay.

The slender stems/trunks are simply suckers, which are new stems that arise from either roots directly or buds on the lower trunk, and a variety of tree and shrub species naturally grow this way. Suckers can help to rejuvenate a "tired" or otherwise ailing plant, as long as the root system remains healthy to support that growth. You can certainly cut away as much old wood as you can to keep the young stems, but sometimes those cuts that create large wounds on the wood don't seal-over well. Still, there is nothing that can be done (it's just a risk you'd have to take if you try it), as no tar/sealant/paint/etc. should be applied to the cut wood, despite products being still sold and recommended for that use.

It's probably going to add stress to the Viburnum to cut back the healthy growth as well, but if it's too tall and you need to manage size, it can be done as soon as the flowers are spent. (Now would be fine if it's not done blooming but you don't mind losing some flowers for this season.) Be sure to not make a pruning cut too close to the junction of whatever side branch you cut it back to. There is no guarantee that Ambrosia Beetles won't infest the young wood as well, but since it's still quite slender (not a lot of room to tunnel in) and vigorous (not in decline), we'd guess it's a low risk. Botryosphaeria canker, though, could affect wood of any age and might still infect the new growth later in its life if the shrub becomes stressed again from drought or other factors.

Miri

You're welcome!