For first time ever, my 2 year old macbook does not seem to work without the cord. Usually I unplug it and it's ok. Now I unplug it and the screen goes black. even with plug I do not see the battery symbol. is this unusual?
I'd say so, yes. Sounds like either a faff battery or a power manager problem.
First, check the battery. Do the lights on the bottom come on when you press the little battery charge button? Remove and re-seat it; does it show up then?