It depends. When somebody comes over, a nervous dog might hide because he/she is not used to the enviroment. Not even a person. When that happens: Try taking your dog for a walk. Bring water, food, leash, collar, and even bring treats (when your dog is being social and good.) Go to a quiet place, like in a park. Walk your dog there. Soon, you can bring your dog to more social places. That can make he/ she get used to things. Thunderstorms: Get a CD if you have a computer or radio. BUT this CD has the sounds of thunderstorms. Place your dog in a clean room with the radio or computer. Play the CD softly, letting your dog hear the thunderstorm. Each day, play it a little bit louder. Maybe your dog is hearing things: dogs can hear things that humans can't. Your dog may be hearing some of those things that scare him/her. Hopefully that'll help. (Maybe you can try being more specific in your question) Wishing you and your Yorkie good luck! 
