Answer: Based on your description I am not certain what has happened. It might be that overly wet, saturated soil and the wet conditioncs have been conducive to a soil borne disease or wilt problem developing. It is difficult to diagnose this type of problem long distance, so you may want to consult with your local county extension for a more specific diagnosis and based on knowing that, they should be able to suggest what to do to treat it. In the meantime, clean up the plant debris and discard it in the trash, you might also isolate the plant from your other hibiscus. Take care not to cross contaminate via tools used on the affected plant. In the meantime, here is a fairly detailed discussion of this type of problem you may find interesting. You may need to cut and paste the complete url into your browser to make it work correctly.
I'm sorry about your plant.
Q&A Library Searching Tips