I came to this forum because of the same problem with one of my hibiscus! Mine is a perennial hibiscus. One that gets very large red blooms. I don't know the exact name but I think it might be a Lord Baltimore. It was blooming fine for most of the summer and then suddenly around the third week in August it started to wilt. At first I thought it was because our weather was so dry and hot. I watered it well and then even after we had drenching rain it has not recovered. I can not see insects on it so I have no idea why the wilting. I am wondering of I should cut it down now in the hopes whatever is wrong does not spread to the one other one I have which is about 10 feet away.

I had the same problem with a double red hibiscus two years ago. I was told by someone on this forum that it was due to a fungus and to apply a weak solution of bleach and water. I was somewhat sceptical but had no alternative and lo and behold it worked!! It is now a huge plant and looks very healthy.