If on Shabbos or Yom Tov the Sechach blew off due to wind may it be replaced?
No. It is forbidden for a Jew to replace the Sechach. This applies even if only part of the Sechach blew off or folded over one may nevertheless not spread it back onto the Sukkah. If however there is no other Sukkah available, one may ask a gentile to replace the Sechach for him.
 M”B 637/1 in name of Shoal Umeishiv