iBook G3 800mhz wont recognize battery

Let me describe what happened first. I was using my iBook, it was plugged in and had like 97% charge in it. Then I looked up at the charge level and it had an "x" I guess meaning it thought that there was no battery in it. So I shut it off and removed the battery and put it back in. I did notice at this point that the battery had a single green light lit on the bottom. I turned it back on and now it showed 0% charge level. It was plugged in this whole time. I then looked up how to reset the PMU and did so. No luck, it now showed that it didn't have a battery in it. I then shut it off and left it plugged in all night. In the morning it still showed that there was no battery in it.

My question is why would it go from being a completely healthy battery to dead in and instant? and is it actually dead or what? I haven't noticed the battery life declining at all recently. I often use it without it plugged in, except when I am playing games on it (which I had been doing at the the time).