The Shulchan Aruch (Siman 8) clearly rules that if a person takes off his Tallit in the middle of Tefila, for example, to go to the bathroom, he must make a new Beracha when putting it back on. This ruling is based on a large contingent of Rishonim (early authorities) cited in the Bet Yosef. They argue that since he was "Doche B’Yadayim"- actively detached the Misva from his self, it is now considered a new Misva with its own Beracha, even though he intended to put it on again.
However, The Rema disagrees and argues that a new Beracha is not necessary. His rationale is that technically, it is permitted to wear a Tallit in the bathroom; therefore, there was no Halachic interruption mandating a new Beracha. The other position claims that this reasoning supports their opinion. Since it is not mandatory to remove the Tallit in the bathroom, and yet he did so nevertheless, this is a clear indication that he was "Doche B’Yadayim."
Halacha L’Maaseh, the practical Halacha is that we follow the principle of "Safek Berachot L’Hakel,"- to be lenient and refrain from saying a Beracha when there is a doubt, against the opinion of Maran.
The Shulchan Aruch brings a second case in which the Tallit completely fell off. Here, even the Rema agrees that he must make a new Beracha, because the Misva, as it were, left the person; he did not actively remove it with intent to put it on again. However, Hacham Ovadia found some Rishonim who held that in such a case one does not make a Beracha. Thus, the Halacha is again a "Safek Beracha L’Hakel," a case of doubt, and we are lenient not to say it.
Interestingly, in both cases Maran rules to say a new Beracha, yet the practical Halacha is not to do so.
Based on this, Hacham Bension rules that if after the Amidah, when wearing a Tallit is less of an obligation, one’s Tallit fell off or he went to the bathroom, he should not put it back on, to avoid putting himself in a situation of a doubt in Berachot.
SUMMARY
If a person removed his Tallit or it fell off, after the Amidah, he should not put it back on. If it happened before the Amidah, he should put it back on without a Beracha.
