Cant save in gameboy advance sp:(

It may well need a replacement battery - what is the game?

Nevermind - I found a list. These are the games that need a functioning battery for saves (the rest used EEPROM or Flash RAM that should still be good);

Baseball Advance
Boktai 1, 2 and 3 (JP)
Bomberman Tournament
Breath of Fire
Breath of Fire II
Castlevania: Circle of the Moon
Castlevania: Harmony of Dissonance
Dragonball Z Collectible Card Game
Hamtaro: Ham-Ham Heartbreak
Kirby: Nightmare in Dream Land
Krazy Racers
Lego Bionicle
Lego Island 2: The Brickster's Revenge
MegaMan Battle Network
MegaMan Zero
Metroid Fusion (Only some use a battery)
Metroid Zero Mission
Monster Rancher Advance
Monster Rancher Advance 2
Pokemon Emerald (Not for saving, but to power the Real Time Clock)
Pokemon Ruby (Not for saving, but to power the Real Time Clock)
Pokemon Sapphire (Not for saving, but to power the Real Time Clock)
Super Dodgeball Advance
Warioland 4

For some the battery is fairly easy to change if you have the right tool to get into the case - for others the battery is soldered in.