Termination of a Case
If the applicant fails to reply to the inquiry correspondence sent by their embassy or consulate, termination of their visa application will begin. The embassy or consulate will first send a Follow-up Letter and Instruction Package to the applicant. If the applicant does not answer within one year, a termination letter is sent. At this point the applicant has one more year to activate the immigrant visa case. If there is no answer in one year, the case is terminated. You can stop termination of a case by notifying the embassy or consulate before the prescribed time period has lapsed, that the applicant does not want the case to be closed (terminated).
Source: U.S. Department of State.