Fatal Error: The script tried to execute a method or access a property of an inc

Last night my site was working fine and today I go to visit it and get this error message:

Fatal error: NF_Forms::get_all(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "NF_Form" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/content/p3pnexwpnas02_data01/50/2068650/html/wp-content/plugins/ninja-forms/classes/forms.php on line 51

My hosting is Godaddy and just downloaded FileZilla to check out the files and such. I don’t have any experience with code but this is what I get on line 51:

$status = Ninja_Forms()->form( $form_id )->get_setting( ‘status’ );

What do i do?