Regarding a Land Issue which has been sold out to someone else.

Dear Sir/Madam. Greetings! I am hereby writing this message to ask you a doubt regarding a land. My father's grand father has a land in Andhra Pradesh which is also registered with his grand father's name as per 1956 land act. Many years has been passed and now its 2020. My father has went away from home in 1998 and he didn't come to our home( As per our opinion he is with some other one but he is still alive living somewhere ). Now, for many years I too dont know that we have 2 acres land until my mother informed me. Recently. After getting information from MRO office in our area after getting information from my mother. I come to know that the land has been sold to some strangers(Currently possessed) by other strangers(Unkown). It took in 1982. I am not sure that my father has sold that 2 acres land to some strangers who sold it in 1982. I have the following questions for the above scenario.Please find below. 1) Many years has passed like from 1982-2020. And I still don't know that we have land until my mother informed me. Please tell me whether I have right to go for appeal on land to claim that If it's illegal transaction or legal transaction without anyone's / my fathers concern. 2) What if the 2nd party who sold the land to the exiting holder without any document(Like my fathers signature or other proofs)? 3) Whether for this any signature or concern required from my father for registering the same in Registration office or stamps office or any govt office(Actually he will be the legal heir for that land)? 4) If the party who is possessing now dont know the illegal transaction whether the sale is valid? Thank you for reading the above paragraphs and please let me know if you require any furthur information for the same. Please help me in this regard. So, that I can proceed further with your great assisstance. Also please you can reply or contact to my email [deleted].