Dear Rabbi Simon,
I have a couple of questions regarding my tefillin.
I have lost the black box that goes above my hand tefillin. Is that a problem?
Also, how often should one have his tefillin checked?
Black box covering: best to replace this. Ask the Sofer down the road from Torah Treasures if he can supply. If you have good quality tefillin and use them regularly they only need to be checked if they have been left in the rain or similar misadventure. The batim should be refurbished and the רצועות replaced when these show signs of wear and tear, flaking, or similar.
Rabbi Rashi Simon