94:26 If a city set up an eiruv and it became invalid on Shabbos, if all the courtyards have proper walls and they have no disqualifying breaches, even if there are many houses in the courtyard, it is permitted to carry there all that same Shabbos (even between adjoining courtyards - Mishnah Brurah 365:39). Even though the eiruv is in another place that has now become separated from this courtyard, it is nevertheless permitted because, regarding this, we say that what was permitted on Shabbos remains permitted. There is a very real concern that many people may carry impermissibly in a place where it's not allowed because they are used to it being allowed, therefore if the eiruv can be repaired by a non-Jew, it may be done. If the cord across the form of a door became torn and it is possible for a non-Jew to repair it by tying a bow or a knot with a bow over it, this is a good thing to do. (A non-Jew may even be instructed to fix the eiruv in order to protect the public from the possibility of sin - MB 276:25.)
94:27 If yom tov fell on Friday and the eiruv became invalid, even though the eiruv is also useful on yom tov to permit carrying non-essential items, we don't say that what was permitted on yom tov continues to be permitted on Shabbos. This is because yom tov and Shabbos are two discrete types of sanctity.