ok so... ive had this problem b4 and ive also fixed it b4.
all games with internal memory have a battery, the more you play the game the more the battery looses life yeah i know common sense..
anyway after a few years of intese playing your battery will die and you will get really upset and say my saved game is gone! wahhhhhh, and then you make a new save game and it will still not save. this is all common with a dead battery..
to the important part! how to fix it. now unless you have a special screwdriver this is going to be a doozy. nintendo has there own patented screw techonology i guess, and these screws are like hexagonal or something practically impossible to get off.. so whats the next easiest thing when you dont have a "nintendo screwdriver" smashing plastic, or ripping the cartridge apart.
along time ago my harvest moon battery died so, i ripped apart another game with internal memory something like mario world 2, and took the battery out from that game.. then ripped apart harvest moon. so now you must carefully remove the battery from the game you want to replace, and i mean very carefully. you see the 2 metal prongs the battery sits on, you must make sure they stay connected to the chip board. now take your fresh battery place it facing the same way as the old one between the 2 prongs.
for this next part you will have to do some soldering, so youll need some solder. if you dont have any handy try using tinfoil, its always a safe alternative for poor people like me. now make sure the metal prongs are connected to the battery with solder (or tinfoil). now carefully put the chip back inside the cartridge shell, if you ripped it apart carefully the chip board should still fit inside and the cartridge should stay intact when you press it together with your fingers. now inset the rigged game with a fresh battery into your gameboy and now you can have a saved game...
any questions feel free to ask away, sorry thats probably way to much to read just to fix a game cartridge.