This is commonly called a rock rose, one of the Cistus, possibly C. x purpureus. Unfortunately, I don't see evidence in these photos about the cause of the death of part of the plant. Provide a photo from further away to show the surrounding plantings, maybe that will give me a clue about what might be the issue.
Also, tell me about how it's grown. How do you irrigate it? Is there landscape fabric over the roots? Do you mulch regularly? When did it first start to fail?
These plants often get too large for their area and owners prune them back. Sometimes the plants recover well, but other times they don't recover after pruning. You might cut the dead parts off and see how the plant responds.
Another option is to cut this one down to the ground and watch for regrowth. If you remove it, take photos of how the roots appear and how they smell. If the area is very wet, root rot is a possibility.

I don’t see evidence of any pest or disease issues even with these additional photos. I found gardening posts online about these plants being fairly short-lived. These are not scientific studies, but ornamental plants are not the focus of many studies, so sometimes antidotal information is the best I can find.
This is one of those “best guess” situations. I think the plant is dying for no reason or unknown reasons, and that you can replace it with a new one if you’d like.