Paying off a loan can cause your score to drop but it should not be that severe and should be temporary.
https://www.experian.com/blogs/ask-experian/why-did-my-credit-score-drop-when-I-paid-off-a-loan/
If you experienced a huge drop for which you cannot account, you should get your credit reports (free from the link below) and review them for any inaccuracies.
https://www.annualcreditreport.com/index.action
