My boyfriend's dog is twitching and will not move its body. What could be wrong?

It was fine earlier today. Then he noticed that the dog's chain was tangled around a tree. When he went to untangle her she was growling and barking at him (which she never does). He got her untangle but saw that she wouldnt move and she began to twitch. She will twitch a while then stop, and then twitch again. He also says her breathing is very slow. What could be wrong with her?