A contractor did some work on my house and to my great horror, a Mezuza was accidentally detached from a door. No one can seem to find it. To say that I am upset that Kavod Shomoyim has been compromised is an understatement to say the least. What can be done, aside from fasting, Tzedaka and replacing it?
B”H that we have people in Klal Yisroel that are so concerned for kavod shomayim, may HKB”H help you in all areas, and that you should be zoche to spread kavos shomayim in the world.
Regarding your question, there is no need to fast, even if the mezuza would have fallen in the floor without the case, however it would be a good thing to give 18 perutas to tzedakah, but it is not an obligation.
Lev Chaim 3-121, Bais Baruch 1-12 (229), Tzitz Eliezer 5-1 (89), Maharsham 2-264, Be’er Moshe 3-9 (8), Daas Noteh 3-pg. 434.