Can dad lose rights fter he gve mom sole legal cutody and disappeared for two years?

Of course if he is gone and you have not seen him for two years and he is not using his visits why bother going back to court.

He will not lose his rights, but the custodial parent may go to court and modify the visit agreement.

He can always go back to court latter asking for rights.

He won't automatically lose his rights. But if mom has full custody, she can block visits until a court has ruled.