The Hibiscus is a great plant for warm and sunny places. From the distance it is hard to judge why it has lost all blossoms and leaves. What I can guess is that the powder you are talking about is probably mildew. This fungal disease often occurs in humid heat. The spores fly to the plant and if the conditions are favourable (weather, microclimate,…) the disease breaks out. There are various remedies for leaf fungus. When you plant new plants, choose more resistant varieties, especially for roses. Also, leave sufficient space between the single plants so that the leaves can easily dry off after rain. But when mildew has already affected your plants, get some advice in a garden center and choose the right herbizide. Also, cut off the affected leaves and branches to get rid of the pest.