Hi,
After not being used for 12 months most probably the battery has failed and needs to be replaced.
Also the reason that the phone doesn't start when the charger is connected is that most likely the charger can't run the phone and charge the battery at the same time.
Here's a [https://www.youtube.com/watch?v=YsC06YBPQiI|video] that shows how to remove the battery from the phone.
Search online for ''CS-NKT811SL battery'' to find suppliers that suit you best.