As everyone has said, blow the cartridge and the cartridge slot on your GBA to get rid of any dust.
Also, make sure the cartridge is in straight. If it's not in straight, then the Gameboy can't load the game. It sounds stupid to even check that kind of thing, but if I had a pound for the amount of times I've stuck a cartridge in and thought it was straight only to find it's at an angle I'd have...well, mabye £10.
But you get the idea. Just check that everything is as it's supposed to be, blow the GB and the cartridge contacts for dust/dirt and try again.