the battery is empty. The older gameboy games (Super Mario, Tetris, ...)don`t have batteries. That`s the reason why you canot save those games. later the games (Legend of Zelda: Links Awakening, Pokemon RBY, ...) got those batts.
you will have to replace the old one with a new one and it will work again.
hope this video explains it, havn`t watched it.