A husband went missing for a number of years.

Answer

If the first husband who had gone missing returns, then the marriage to the second 'husband' will automatically terminate. The second man will not remain her husband since her true husband has arrived. She will remain the wife of her first husband. However, the first husband may not have conjugal relations with her until the expiry of Iddat. Since she had relations with the second husband, she has to observe Iddat.