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It is in shock. They need to be acclimated when brought inside. This article will help with that for next time:
https://www.gardeningknowhow.com/houseplants/hpgen/using-pesticides-and-other-chemicals-on-your-houseplants.htm

It should recover just fine. These plants are prone to losing lots of leaves indoors in the winter anyway, and always come back from that. Keep it correctly watered and you should see new leaf growth.

Hardy hibiscus does not need any winter protection in your area and should not need any special protection.

This article will help with pruning:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm

If they are not blooming, they are either not getting enough light or not enough phosphorus. If the plant is lush and green with no blooms, it is likely lacking phosphorus. Give the plant some bone meal to supplement it. If the plants grow slowly and seems sparse, it is likely light and they will need to be moved to a brighter location to bloom.

While overwintering indoors is always preferrable, especially for tropical varieties, one of the best ways to protect outdoor hibiscus that are planted in the ground is to mulch over the root zone and around the main stem of the plant. For additional protection (such as when extreme temps are expected), wrap the entire hibiscus in heavy frost cloth, tarp, or burlap. And yes, it definitely helps to prune back the plant but only AFTER winter, as the foliage will help insulate the plant. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm

This is a common issue with hibiscus plants. The following article should be of some help to you: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm