The first thing you need to do is get a copy of the order or other document which authorized the garnishment from the bank. That document will at least let you know why the garnishment was implemented, and may provide you with some information on how to challenge the garnishment. (Or did you get some documentation somehow indicating that the garnishment was for your father-in-law's debt? If so, review that documentation.)
Information on the procedure that is supposed to be followed for a garnishment authorized by a court is set forth in
Rule 64d of the Utah Rules of Civil Procedure.
You may also wish to check with a local lawyer to see what duty the bank has under Utah law to notify you of the garnishment, or if the bank can simply assume that you have received notice from the garnishor.