I recommend checking to see if you can get the battery replaced under warranty. Otherwise it is time to buy a new battery.

Supposedly you need to cycle your battery at least once a week and it will last a long time. The same happened to me, after about 6 months my battery wouldn't hold charge at all. I've never replaced it since I don't actually use it outside much, and didn't want the same thing to happen again. Although, now ever since I've been hearing the cycle 1 once a week method, I might look into buying another battery. I suggest looking at the reviews on the apple store for the product, where some people offer their opinions on how to get the battery to hold charge for a long life.