What happens if your tallis falls off, is a bracha required?
Both according to Sephardim and Ashkenazim it depends on whether or not you realized it was falling off, and thus had intention to put it back on immediately or not. See Shulahan Arukh 8:15, and the Mishnah Berurah, as well as the Kaf HaHaim there , and Ben Ish Hai Year 1 Berashit 10 .
The reasoning behind the Kaf HaHaim and Ben Ish Hai is that a second Berakha would be a Berakha sofek l'vatala, and thus should not be said. However, if it falls off without your knowledge and there was no immediate intention to put it back on then it appears another Berakha is required.
If one's tallit or tzitzit fall entirely off, he must recite a new blessing before putting them back on. This is true even if he picks it up and puts it back on immediately.
Among Sephardic Jews, one who removes his tallit intentionally must recite a new blessing before putting it back on. Among Ashkenazic Jews, though, one who intentionally removes the tallit need not recite a blessing before putting it back on, so long as he did not travel far and did not wait a long time before putting it back on. It is difficult to define "a long time;" ask your local authority for more information.
(Code of Jewish Law Orach Chaim 8:14-15; Mishneh Berurah 8:40)
from Mordechai - http://halachahbyemail.blogspot.com/2010/11/if-tallit-falls-off.html