It's quite possible that the battery is finally dead. Batteries don't have an infinite life span for charging. Just because a light comes on doesn't mean it's actually holding the charge.
When you turn the computer on, is it still plugged in or are you trying to boot it up in battery only mode?
It's an IBM so if has to be old. IBM no longer makes personal computers. So exactly how old is it? Sold its manafacturing wing to China. Given the age, it could also be another hardware issue as well.