I have had my hibiscus tree for 8 years – summer outside, winter in. It has been beautiful tree all these years. I was away for 3 months, had someone looking after my house and plants. But on the coldest day (-40C), the furnace went off. I came home 3 days later and my poor hibiscus tree looks dead or close to it. All the leaves are going. Is there hope?
I would continue to care for your hibiscus as you have for the last 8 years.
The plant should be still alive if it stayed above freezing 32 degrees.
Avoid moving the plant much, as this can stress the hibiscus.
Here is an article about hibiscus care.