Been charging for over 8 hours, light indicates still charging

So absolute disclaimer; This calculator worked fine in high school for me. However, it’s been around 4 years and as such the calculator has been sitting in a box unused for that time. I’m trying to see if it still works to sell it. At the time I’m posting this it’s been charging with the amber light indicator for 21 hours. Over this time I’ve tried turning it on multiple times with no luck. Should I let it continue to charge until I get a green light or is something wrong?