Fallen Tefilin

Question

My Tefilah shel rosh fell on the floor. Well, on the carpet. It fell without the covering. I was in a siting position, and one of the straps got hooked under my shoe. When I went to lift up the tefilah, it slipped out of my hand and fell. I gave tzedaka. Do I have to fast? It's tough during the long summer days.......