When you are deactivating 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 as 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 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.