Hmm... did you try blowing into the cartridge and the game slots? Dust or other debris that gets caught between the connectors can make a system fail to recognize the presence of a game. It drives me crazy on my DS or when I'm using a GameCube controller on my Wii...
Also, did it work prior to now? If it didn't, then you might have gotten the cartridge defective and you should demand either an exchange/refund. If not, then it's possible something might have been damaged inside of the game cartridge. If it worked before, shake the cartridge around gently and listen to hear if anything rattles on the inside (NOTE: the only thing that should make a noise is the main circuit board on the inside moving around a little). If there's a second rattling noise, then the cartridge is most likely broken and you'll have to get a replacement.