Battery won't charge? Casio np-20 lithium ion?

I've been trying to charge my digital camera's battery for days now, and it won't charge. My camera is a Casio Exilim EX-Z60, and the battery is a Casio NP-20 Lithium Ion battery. Does anyone have any idea what could be going on? Would it be the charger or the battery with the problem? Should I get a new battery?