We live in Leesburg, Florida, about 40 miles north of Orlando. I have 2 hibiscus plants on the east side of my house; they get lots of sun. Problem is they're both losing all their leaves. I fertilize regularly with "hibiscus fertilizer" and water twice a week. They were planted 2 years ago and did well for the first 20 months.
A sudden loss of leaves usually indicates shock. The shock is caused by a sudden change in the plant's growing conditions. Overfertilizing, sudden changes in the air temperature, sudden changes in watering, and sudden changes in the amount of sun they get are often factors.