Sometimes it becomes apparent during Shabbat that a wire from the eruv has snapped in a certain place, thus invalidating the eruv. Two questions then arise: 1) May the eruv be fixed on Shabbat? 2) If it turns out that the eruv cannot be fixed, do all the residents need to be informed that the eruv is down so they know not to carry?
Le-khatĥila, if a non-Jew is available, it is preferable that he fix the eruv. Although asking a non-Jew to do melakha for Jews on Shabbat is rabbinically prohibited, nevertheless, when there is a great need to save the masses from the prohibition of carrying on Shabbat, the Sages permitted asking a non-Jew to fix the eruv, even if this will involve melakhot that are prohibited by Torah law (MB 276:25; above 9:12).
However, if no non-Jew is available, a Jew certainly may not violate Torah prohibitions in order to fix the eruv. For example, if a pole has fallen over, a Jew may not stand it back up by jamming it into the ground. Similarly, if the wire has snapped, one may not tie it back together with a permanent knot, as tying this type of knot on Shabbat is forbidden by Torah law. However, the poskim debate whether one may tie a bow knot (the type used to tie shoelaces), as tying this type of knot on Shabbat is permissible.
Some argue that fixing the eruv on Shabbat is absolutely forbidden. Even though it is permissible to tie a bow knot on Shabbat, nevertheless in this case tying it would permit carrying on Shabbat. This means that by tying it, one constructs a meĥitza ha-materet (above, 15:4), which is forbidden on Shabbat. In other words, the Sages prohibited making a wall that serves to permit something that was previously halakhically forbidden. In this case, before one tied the knot it was prohibited to carry, while afterward it was permitted.
Others maintain that although the Sages generally forbade constructing a meĥitza ha-materet, nevertheless, in order to save the masses from sinning by carrying on Shabbat, one may create a meĥitza ha-materet by tying a bow knot. This is the common custom (Responsa Mahari Ashkenazi §13; Panim Me’irot 1:30; Sho’el U-meshiv, Mahadura Tinyana 1:89; SSK 17:34).
If the eruv cannot be fixed, this should not be announced publicly. This information is withheld out of concern that some people will carry anyway, desecrating Shabbat knowingly, and it is better that they transgress unknowingly rather than knowingly. The only people who should be informed are those who will definitely follow the halakha and refrain from carrying.