It will likely depend on how cold and the duration of the frost.
Cold damage may not be apparent right away, as the plant tissue will continue to decline over a period of time.
Make sure you move the plants indoors whenever the temperatures drop below freezing, though to be safe I would even consider keeping them in a sheltered area if the night temperatures are not above 40 to 50 degrees F.
Trim away any dead material and continue to give your Hibiscus good sunshine and care.
Do not fertilize until you see active growth on the plant.