Leaves turning brown
The leaves on my Plumbago are all turning brown causing a mess to say the least.
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.