Ray, Hillary is a private citizen. She is no longer the Secretary of State. She has rights as a citizen that sure as hell don’t authorize one committee chairman to get hysterical and call for her arrest and imprisonment, as a civilian.
You have a lot to learn, if you are really serious, and are not just a beat your chest screamer type.
The fact that she is a private citizen is immaterial. An investigatory committee of Congress possess the authority to issue subpoenas. This authority has been confirmed by the Supreme Court. See Anderson v. Dunn, 19 U.S. 204; Jurney v. MacCracken, 294 U.S. 125; McGrain v. Daugherty, 273 U.S. 135.