Our son and daugher-in-law owe us money that was loaned to them. I had a loan folder that contained all the information. The day my daughter-in-law said and made up the fact that that loan had been forgiven I found out that the loan folder was gone. Therefore I have no proof. The amount is $16,534. Do I have any way of getting my money back?

If you have a record of the original payment then you can sue them forcing them to prove what they have paid. Unless they have a document showing the loan was forgiven, they are unlikely to win.

Yes, Of course you should call and talk to them and consider filing theft charges for the folder and contents.

Next you have I assume proof of the payment, get a copy of the check from the bank.
You then send them a bill for the estimated remainder, using the check as proof of your loan to them.
You can show a copy of bank statements showing regular deposits up to the point they stopped paying. As proof they were paying this back.

They will have to prove it was fully paid, with other cancelled checks.

Also your police report filed can be used as evidence to show they made a effect to hide proof of the debt