Was it a loan or a credit card? If it was a credit card, did you close the account?
Closing the account reduces the amount of credit available to you, increases the percentage of credit you are using when you have other debt, and decreases the average age of your accounts/history and can cause your score to drop. Tho I think it's supposed to remain on your report 7-10 years so the age thing might be less of an issue.
So if you have 3 credit cards with a 1000.00 limit, 2 have a 500.00 balance and one has a zero balance, if you close one account with a zero balance, you go from using 33% of your available credit to 50%, and if the account you closed was 20 yrs old and the two you kept were 10 years old, you've reduced the average length/age of your history. At least that was the gist I got from reading about this a while back.