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You don't say how long you've had the plant, or what the sun/shade set up is like, or the soil condition, or the history of the plant. Here's an article that covers general plumbago care: https://www.gardeningknowhow.com/ornamental/shrubs/plumbago/growing-plumbago-plants.htm
If the plant is established, and suddenly turned brown, see if anything in the soil changed, keeping the soil too wet, or keeping water from the roots. Has it been unusually hot? That could make the leaves brown. Check to see if the branches are dead -- if they snap when bent, they're dead, if they bend they're still alive. After these points, consider insects or disease. I think your best option is to enlist some professional help to analyze the plant, leaves, and soil. You can contact the nearest botanical garden for help in finding a master gardener to advise you.

This article may help: https://www.gardeningknowhow.com/plant-problems/environmental/fixing-color-faded-flowers.htm

Plumbago is such a tough plant, it's hard to imagine what could cause an established one to die back, unless it's cold or poison. I don't think your winter was cold, was it? Are you sure it couldn't have been hit by herbicide? The stuff can drift quite a distance from where it was sprayed, and it can also wash through the soil for several hundred feet.
If you use a lot of water on it, you might be able to wash bad stuff out of the soil, if that's the problem.
Can you take soil and leaf samples to be analyzed by a university or agricultural lab? The most you can do, unless you can identify a disease, is to cut back dead material, and hope for the best.
I love plumbago. Good luck to you.

Please don't do anything hasty ... it's not the plumbagos' fault. I think the best thing would be to scrape off the moldy mulch, and replace it with fresh clean mulch. This article has some general mulch information for you to consider: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-flowers-gardenia-buds-falling-off-plant.htm

I've never seen any indication that the color of plumbago can be affected by soil chemistry. The color is determined by the variety of plant. If your heart is set on a white flower, I suggest you contact the store you bought it from, and explain the problem. They should, at the very least, give you another plant. If they don't, at least you'll know never to deal with them again, because any reputable plant store should provide a year's guarantee to their products.