That generally means that the computer has determined the battery is bad, and that it can't charge the battery, or draw power from it. I'd download Coconut Battery:
This will tell you the capacity of your battery, load cycles, if it's charging, etc., and generally give you a clearer idea of what's going on.
Although it's possible your computer has hardware issues that are not letting it charge the battery, chances are that your battery has simply gone bad and you need a new one. Try a friend's battery with your computer in order to confirm this.