Does anybody know why all of the sudden mu battery icon shows "charged" and the plug-in sign?

I have not had my MPB plugged in for the last 2 hours. However something new has occured. The battery icon up near the time shows the power adapter icon in the battery but it is not plugged in to power and it shows "charged" but it can't be charged.

any suggestions on why this has just started and what to do? Isn't there a way to try and reset the SMS (I think it is called).