Hi Ben, Tropical hibiscus love sunlight and need it to stay healthy, bloom and keep on growing. They can survive inside for short periods, but for many reasons, cannot survive easily inside.

A root or collar rot fungal infection is most likely the problem. It is nearly always present in soils and most potting mixes and is kept at bay by a healthy plant grown in its required conditions, specifically minimal water to match plant needs. It sounds as if your mulch has done its job ie keep moisture in, and fungus has then had ongoing moisture to get a hold of the plant's wet rotting roots.

You may be able to save it by putting out in dappled light, out of rain, remove mulch, let it dry out a bit, cut back some foliage, say a quarter and treat with a fungal chemical. As health and plant improves, hopefully, give it morning sun and so on til full sun til midday. Feed it with some good quality compost water and if it has reached this stage, it should keep growing.
Re-potting is an option if above process is not able to save plant.
Good luck, Brian.

Water with a couple of tablespoons of chlorine washing bleach to a gallon of water and then let it almost completely dry. If it is sitting in a drain saucer, remove it. They don't like wet roots. The next time you water, fertilize (liquid) it, add Superthrive and let it almost dry out again. Lots of light and warmth will help it revive.