« je n’ai plus d’argent » sous-tend que j’en avais auparavant alors que « je n’ai pas d’argent » peut vouloir dire que je n’en ai jamais eu.

Je n’ai pas d’argent

means you have no money. Maybe you had some money before, maybe you have never had any money – but right now, you have no money.

Je n’ai plus d’argent

means you have no money left: in the past, you had some money, but now you have spent it or you have lost it, and you have no more money now.