First, expressing credit score in terms of a percentage really has no meaning. The credit score is simply a scale, and it is a secret scale: it may or may not be linear. That having been said, I agree that a drop from 815 to 675 is a lot. There are several things for you to look at.
There are several different brands of credit scores. FICO is the first and most well-known, and most used by lenders, but there is also Vantagescore and others, and they each have their own secret formulas. The non-FICO scores are attempting to be close to FICO, but they are not the same formula. Often, people will use the term FICO score as a generic name for credit score, but they may not actually be looking at a true FICO score. Make sure that your before and after are using the same scoring formula.
Even if both your before and after are using the same brand of credit score, they periodically change the formula to generate the score. For example, FICO just recently released FICO 9, which is different in several ways from the previous FICO 8, including using a different maximum and minimum score for the scale. Check to see that the scores you are comparing are the same version of score formula.
Your score is calculated solely on the items in your credit report, but you have three different credit reports, one from each of the three credit bureaus (Transunion, Equifax, and Experian), which means that the same score formula can give you three different scores depending on which credit report is being used. Make sure that the scores you are comparing are based on the same credit report.
You can get free official copies of each of your credit reports by going to AnnualCreditReport.com. This site allows you to request one free report every 12 months from each of the three credit bureaus. If you want credit reports more often than that, you can pay for them.
LexisNexis is not one of the three credit bureaus that your credit score can be based on. LexisNexis does have a financial data file on you that is somewhat similar to a credit report, but it is not used to calculate a credit score.
My recommendation to you is to pull a new copy of each of your three credit reports and look for anything that could be pulling down your score. Perhaps you will find something on a report that can explain the drop in score that you saw. If you see anything on any of your reports that you believe is not accurate, you can dispute the incorrect data. Specifically, late payments or large balances would be the negative items that would have the biggest impact. Closing accounts will affect your credit utilization percentage, but will not affect the age of accounts right away, because accounts stay on your report for 10 years after they are closed.
If you are really concerned about your score and don't see anything on the reports that explains it, you can pay for your scores directly from FICO. However, I will warn you that it is quite expensive.
Finally, even if you come to the conclusion that your 675 score is accurate, consider the possibility that this may not be as important as you think.