It's really hard to tell what's wrong without knowing what type of tree it is. For some trees this is a common reaction to being moved and is nothing to worry about, but for other trees it means that it is the end of the road. Could you post a picture so we can ID it? If it does die maybe you can take it back to the nursery and get it replaced. I don't think there's anything you could have done to kill a tree in a week.
Also,a tree without leaves will not need as much water as a tree that has a lot of leaves. Usually watering on a schedule doesn't work as well as watering based on the dryness of the potting mix. Check out the watering tips in this thread https://www.helpfulgardener.com/forum/viewtopic.php?t=1479
Good luck! I hope your tree pulls through, and I hope this trouble hasn't put you off to bonsai.