Create a new custom form with the same message class name and same custom
form fields and publish it to the folder (layout is not significant) and
you'll see the data within the form since when you open the contact it will
look for the message class (custom form) that created it. Even then, you're
data is not lost. Without recreating the form - you'll find the data in the
contact record under "All fields" --> "User-defined fields in this item"

Karl

If you forgot the name of the custom form (message class) - create a view
that shows the message class.