Credit Scores
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@Tapxe mentioned above, this is most likely the reason for the drop, assuming it was a credit card and not a charge card (meaning, it had a credit limit like a Visa or MC and not monthly full payoff but no set credit limit like the traditional AMEX cards).
The biggest component of your credit score is credit utilization. It's a simple calculation of your outstanding credit balance against your available credit. So, if you had two cards, each with a $1,000 credit limit and your outstanding balance was $500, your utilization with both cards open would be 25%, which is good because it's below the magic 30% number, but with one closed, you're now looking at 50%, which is bad because it's above the 30% threshold and you get whacked really hard when the utilization is above 30%.