I have enjoyed my blue hibiscus for 11 years . Last year I had lots of flowers but few leaves, this year it is totally bare. I live in upstate N.Y. we have had a very wet year. Is it possible it is dormant as it started getting leaves early spring and then weather got extreme.
Check the branches. Do they bend or snap off? If they have died off, they sill snap off.
Excess moisture can be an issue, especially if the soil does not drain well.
The leaf drop is likely due to stress, and whether the shrub can recover will take time. You may need to wait it out until next spring.
Pull back any mulch and check the soil moisture.
These links will refresh you on the care requirements.