Well, Hibiscus respond to varying stresses often by losing their leaves. A temperature change, even in the garage, can cause the leaves to fall off. No water is stressing the roots so that will cause the leaves to fall off. And a dark environment may also cause the leaves to fall off.

I would, first off water the poor dear. LOL Then you should put a plant light over it for heat and light. Give it a photoperiod of maybe 16/8 light then dark respectively. This should induce some growth for you. (If it has not gone too far from recovery). Trim off anything that is dead so the plant can focus its energy on the healthy part. And give it a little food too. Not much fertilizer mind you as the water itself will probably be absorbed quickly. Make sure you use a fertilizer that is moderate nitrogen, very little phosphorus and more potassium. A fertilizer combination of 17-5-24 should work the best. Feed it a little each week instead of a large dosage at one time.

From there, you can only hope that it recovers. Sorry to hear your plant is stressing. I hope it makes a comeback!