Hibiscus just do that. The bottom leaves will fall off on a moment's notice for no reason at all. They are temperamental about being moved, so it was probably just the move indoors that did it.

But the entire plant has lost its leaves; thosee that are left are wilted and dry. It looks like it hasn't been watered in months, yet it has. Do I need to repot it? Or just leave it alone and wait for spring to see if it will bounce back?

When you say the leaves have withered, are they soft?
Also, if you can, pull the plant out of the pot and see if the soil farther down (more than just a few inches) is wet.
I've had hibiscus whose leaves have shrunk up, but aren't crisp like they normally do when they die off.
If this is the case, your hibiscus, like mine did, could have a fungus from overwatering. It's very easy to overwater them once they're indoors.
I'm going to look for an article I read that explains how to stop the fungus. It worked for mine.
Barb

Hibiscuses don't like wet feet. When they get root rot the leaves wilt like they are dry but they are soft and pliable. I would remove the watering bulbs. If they are sitting in a pan of water, that is a NO-NO! Water only enought to keep them barely damp. They will lose most leves in winter, even in a greenhouse.