Hi Ellie and welcome. Is this the plant you're talking about? What a pretty thing! A picture of yours would be very helpful - yes, we need to see what is going on no matter how bad it looks.
If it is, well impatiens have had onging problems with disease in the last few years, and it may have come to you already infected. If that's the case, I would try a douse with a mild hydrogen peroxide solution - 1oz of drugstore H2O2 to a quart of water. But don't hold out much hope if it is already in trouble.
You could try taking some cuttings if there are any green stems with leaves still healthy. Impatiens start very easily from cuttings so you may be able to get a new plant started. Hard to say whether a cutting will still carry the infection though. If it does, it probably will die instead of rooting. Again, try a few drops of H2O2 in the cutting water.
