Add a new dummy case, select add this as subcase in the 'Add case type' wizard, it should show you an autocomplete control to select the parent case, select the case which you are looking for as a parent case. You should see the new case created under your parent case in case explorer. Remove the dummy case which should display individually in case explorer and not in parent case tree, later you could delete the dummy case.
I beleive you had created a case type which has been removed unexpectedly from case explorer. To add that again click on the Cases Explorer menu located to the right of the Case Explorer and then click on add a case type. Now you will be able to add a new case type or an existing one.
Please try the above mentioned approach and let me know if it helps.