I would be surprised if you have been able to cause any serious damage to this Hibiscus just by feeding it with ericaceous plant food. However, could it be possible that it might have been over-fed, over a period of time?
I don’t want to get to scientific, but when a plant is fed too often a process called Reverse Osmosis can occur. Osmosis is best described as the movement of water from an area of weak salt concentration to an area of stronger salt concentration (from soil to plant). When plants are overfed the balance swings and the salt concentration in the soil becomes higher than in the plant(s). Reverse Osmosis therefore occurs, the water moving from the roots to the area of higher salt concentration in the soil. The result can be wilting and death. The rain this evening may save the day, as the only remedy is to water, water, water to try and dilute that salt concentration in the soil.
Hope I’ve helped,