When you deactivate a form, the status of that form changes to Deactivated. Also, it doesn't appear in the forms listing by default (since it is considered an inactive form).

To find the inactive form you need to reactivate, apply a filter for inactive forms in the forms listing, then select the form that needs to be reactivated. Now, follow the steps to change the status of that form. Once the status of that form is changed from 'Deactivated' to some other status, it will start appearing as an active form in the forms listing.