Answer: This will depend partly on how damaged it was and how stressed it is. It is pretty difficult to keep a tree long term in a container, but you can try. Another reason for concern is that garden soil is not suited to container use. So I hope you used a good quality potting mix formulated for container plants (so it will hold both air and water and drain well yet stay moist) and that the tree had not been planted in the ground for very long.
If it has died back, trim away the dead branches. (I hope it is still alive -- a live tree may lose its leaves but the branches will remain supple. Dead branches are usually off color dull gray or brown and snap in your hand. Live wood has green inside the bark, dead wood has no green inside.)
Then water as needed to keep the soil slightly damp, just moist like a wrung out sponge. It should not be dried out but it should never be saturated or sopping wet, either. When you water make sure it is soaking down into the soil and not just wetting the top layer. You need to check on the soil moisture any time it is not frozen. In rainy weather and cooler weather you may not need to water it or water as often as in hot dry weather.
Your container needs to have holes in the bottom to let excess water drain out, too. It is a good idea to raise the barrel up an inch or two off the ground. This will allow air to circulate under it, keeping it cooler in summer and also helping prevent the bottom of the barrel itself from rotting away due to excess moisture.
I would not fertilize the tree now, wait until spring and use a top dressing of good quality compost along with a light application of slow release granular fertilizer such as 10-10-10. Read and follow the label directions. You do not want to overfertilize as this will make it outgrow the barrel, you are fertilizing only enough to keep it healthy.
I would set it in a location with gentle morning sun or bright dappled light all day and protect it from wind. You may not know whether or not it is okay until you see it growing again next spring. Good luck!
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