Hi,
After 6 months of no use the battery may have become depleted beyond recovery and may need tot be replaced.
It may be that the charger can't do both i.e. run the tablet and charge the battery at the same time.
Here's the ifixit [guide|32139] guide that will help.
Replacement batteries are available online. Just search for ''BT-A0B2 battery'' to find suppliers that suit you best