If that were mine, I'd cut the stems back to the crown and root prune hard (at least half the roots), spray off every bit of soil around the root ball then soak the root ball in a half strength solution of insecticide fungicide for an hour or two then pot it up in fresh soil and use the half strength solution to water down the soil. It's 50/50 whether or not it will survive. In my experience this treatment has saved a lot of mystery sick hibiscus.
Healthy pot bound hibs actually respond very well to this sort of treatment but instead of insecticide fungicide I use a half strength fertilizer.
