Is it possible that smallpox comes back?

Agree with Dr.Raff. It won't come back naturally as it has been eradicated. The American public has not received routine smallpox vaccinations since 1972. All natural sources of smallpox have been eradicated. According to the CDC, “The U.S. government has enough vaccine to vaccinate every person in the United States in the event of a smallpox emergency.” This is part of Emergency Prepared & Response.
Not likely. Not sure if you are asking whether the virus will make a comeback or whether having recovered from infection it can recur. Assuming the first, there are stored specimens of this virus which could potentially be used as biologic warfare agents. The argument for complete elimination of these has been going on for years.