Are you returning the value, label or an array for the checkbox field? This information would be needed to write code.
The example here on showing displaying the value and label is old but it has the basics https://www.advancedcustomfields.com/contact/
What you need to do is get the field object. Then instead of looping over the values that are checked you loop over all of the values in $field['choices'] and you display them all and you check to see if that value has been set in the field value, if it is you show one thing and if it’s not you show another.
