Batteries not available...yet its running...

I have a mbp (about 3 years old) and suddenly I have this icon on the menu bar, it is a battery with a cross on it and it says "batteries not available" For some strange reason I am currently using it off the battery, as I type this. It seems even stranger that I cannot charge it when I plug my charger on the machine. I just get a green dot instead of the amber one, which means that it is charging the battery. Any help on how to fix this?

It may be that your battery is on its last leg. Download this free battery application: coconutbattery and post the results back to this same thread.

That was strange, I just reset the PRAM and the SMC and it worked. That app is pretty handy, thanks again!