How was the Greek nationality lost?
Each way of losing Greek nationality has its own route back, and two are declarations you are entitled to make. Taking another citizenship never ended Greek nationality on its own, which is why the first answer is there.
Someone naturalised abroad (became American, Australian or similar) A woman lost it by marrying a foreigner, before 1984 A child of a Greek mother lost it on recognition by a foreign father It was given up on purpose (renounced or released) Stripped under article 19 for leaving Greece (1955 to 1998) A Jewish ancestor lost it around the Second World War I'm not sure how it was lost