what is the opposite word for block ? is this Unblock ?
If you mean "block" meaning "prevent access", then yes "unblock" is used.

If you mean "block" meaning "put blocks in place" (like in construction), then I don't think there is a simple opposite. Probably just "remove blocks".
i wrote a application in which i had to mentioned to one thing ... i blocked my atm card when i lost it. now i got it , it was saved in my drawer . so i wrote a app. plz unblock my atm card. is it ok ?