Before you decide to give up on your battery and get a new one, there might be a chance that you finished using up every last ounce of energy in your battery and it didn't get time to charge again :)
Your laptop generally tries to charge the battery + run on power when it's connected to a power adapter. What's happening now is, the battery is not getting charged (actually, it's not getting detected) and your laptop thinks there's no battery and is simply running on power.
A new battery will of course have some charge and your laptop will be very happy with it and keeping charging it.
This is the 3rd time I'm writing this instruction today:
> Remove the battery and kill the static energy in your laptop by holding down the power button for 20 seconds.
> With the battery still removed, plug in the adapter and turn on the power.
> Your laptop starts working now.
> Now here's the tricky part - Insert the battery 'after' you've powered on the laptop. Now your laptop will detect the battery and start charging it.
Hope this helps! :)