My tefillin fell on the floor what am I supposed to do?
This is sad because the tefillin have tremendous kedusha and when they fall on the floor it is a disgrace to them. Practically though, it depends on how they fell. If they were inside the tefillin bag, or even if the bayis was inside its cover, then it suffices to give tzedakah, and there is no need to fast. If, however they fell directly on the floor then it is worse. Then, there are people that fast, that day, others do different things as a form of teshuva, such as fasting half a day, and giving tzedakah, a taanis dibbur, others make sure to learn an extra hour or two. This depends on who you are, and what you can do.
Additionally, there are those who make sure to do something to improve the way they relate to the mitzva, such a learning the halachos, or giving the tefillin in to be checked.
M”B 40-3, Piskei Teshuvos 40-2.