38 CFR 19.32: Closing of appeal for failure to respond to Statement of the Case.

The agency of original jurisdiction may close the appeal without notice to an appellant or his or her representative for failure to respond to a Statement of the Case within the period allowed. However, if a Substantive Appeal is subsequently received within the 1-year appeal period (60-day appeal period for simultaneously contested claims), the appeal will be considered to be reactivated.