How do you approach a withdrawn child?

As I said before I am raising the 9 year old child I had as a 18 year old, because her mother died in Texas last year and one of the mother's friend just dropped her on my doorstep here in Utah and left her here. She didn't know about me and my wife and 5 and 1 year old sons, but soon settled into the idea of having a papa and mama and little brothers along with getting use to her new home and learning Russian(my wife tutors her). I thought all was well until she began school here in January.Though a good student she was mouthy and defiant. And trouble began at home as well with her throwing tantrums, refusing to listen, and talking back. Someone suggested that maybe it's belated reaction to her mother's death, so I told her that if she wanted to talk about her dead mother. She said 'NO! Mommy's dead and leave her alone!' Ok since it clearly wasn't her dead mother who troubled her I asked if anything else was wrong and again she shouted NO. Since she claimed to be fine I decided to put my foot down, because no child is running my home! I took away her toys and put them up in my own closet. My wife caught her attempting to take them back and asked her to get down at of the chair and leave the toys where I had put them. She pushed my wife and continued until I myself came and physically took her from that chair. I told her, "If you do not leave those toys where they are and stay out of private space you are getting a spanking." I put her down on her feet and she immediately went to the ground in a tantrum. I picked her up and stood her in a corner saying she would get no attention until she stopped that tantrum and calmed down then I walked away. Problem is she refused to stay put. When I ordered her back she refused and when I carried her back she left as soon as my back turned. After 5 minutes of this I had had enough. She flopped on the floor to continue with the tantrum I picked her up again, but not to return to the corner. I busted her butt and put her to bed screaming. It's been 2 days since then and I am honestly worried. She isn't the same child. If it isn't time for a meal or Russian lesson from my wife the child remains in her bedroom and doesn't make a sound. I wanted her to mind, but this is unnerving me!