The question was recently asked regarding Hilchot Mezuza: If a person discovered that a Mezuza fell off his doorpost, must he recite a new Beracha when he reaffixes it? This would seemingly be analogous to the Halacha of a Tallit which inadvertently fell off a person’s body. In such a case, the Shulhan Aruch (8:15) rules that he must make a new Beracha when he puts it back on, since the Misva has left him in the meantime. The Pitheh Teshuva (Yoreh Deah 289), as well as Rav Shlomo Kluger, the Aruch Hashulhan and Yismah Lev Yagen do, in fact, infer from this Halacha that one would make a new Beracha on the Mezuza.
However, Hacham Ovadia argues that the case of Mezuza is not analogous to the case of the Tallit. In the case of the Tallit, once the Tallit has fallen off, there is no obligation to put it back on. The original Misva has been interrupted with a Heseh Ha’Daat. If one chooses to continue wearing it, it is a new Misva and therefore requires a new Beracha. On the other hand, the Mezuza is a constant obligation on the house, and reaffixing it is a continuation of the original Misva that does not require a new Beracha. Moreover, Hacham Ovadia cites Rabbenu Yonah who holds that even in the case of a Tallit that fell off a new Beracha is not required. Based on these two factors, he applies the principle of "Safek Berachot L’Hakel"-to be lenient and not recite a Beracha in a case of uncertainty. He originally presented this discussion in Yabia Omer YD 3:17, and then forty years later in Halichot Olam (Vol. 8 in the footnotes of p. 268).
Hacham Ovadia also discusses in which a person intentionally removed the Mezuza in order to replace its case. In such a case, no new Beracha is required, unless he kept the Mezuza off for a few hours, until he took his mind off of it, constituting a Heseh Ha’Daat.
SUMMARY
If a Mezuza fell of the door post, a new Beracha is not recited when reaffixing it. Similarly, if a person took off a Mezuza, in order to change its case, no Beracha is required when reaffixing it, unless he delayed reaffixing it for several hours and took his mind off of it.
