Hi- There are some great programs (ask your lender if he/she can do this for you) that tell you specifically how you can increase your credit score. Sometimes it's a matter of paying off one card by transferring your balance to another. You might be able to improve your score very easily.
Some times when you close an account the score can drop but not usually 100 points, the key is whether you were current when you sold or if you were behind. You should meet with a loacl and tusted mortgage broker that can pull all 3 credit files and compare them for you. Good luck with your mortgage.
There is likely more to this than simply selling your home and your credit score dropping 100 points. A drop in this amount without documented reasons should definitely be investigated further.
Leave no stone unturned.
Were you current on your mortgage the entire time?
Often when an account is closed it can hurt your scores, usually it's just a few points, maybe even a dozen, but 100 points is very severe. If there aren't many other accounts on your credit report then closing an account tends to have a bigger negative impact on your score.