Yesterday, some time Shabbos morning, someone noticed that the eruv was broken. It was fine on Friday night but was not up in the morning. And (as I understand it), they told the eruv rav, who sent out messengers to the various shuls that it was down.
Naturally, this caught many unawares -- those who were already out carrying, those in shul with keys to their house and no Shabbos belts, those who had almost no experience with no eruv on Shabbos, and so on. A while later, somehow, the eruv went back up, and messages went out to inform people -- though some shuls had already let out.
It raises interesting questions. Should the fellow who noticed this just have kept silent, and let people continue carrying in the karmelis? (One rav purportedly said so.) Do you tell someone who is already out carrying something or pushing a stroller? (It was suggested not to. If informed, what do you do?) If when Shabbos came in, the eruv was up, why not extend the permitted status throughout all Shabbos. (There are discussions about this in what I link to) How can a partition of the tzurat hapetach be erected on Shabbos, and take effect? Who directed this fix? Was it a Jew, bemeizid? If this is not really building, is such permitted? Was what fell down the electric line or telephone line and the company came and fixed it? And does the reerection of the eruv bring back the initial permitted status?
It has been a while since I've seen any of these issues inside, in depth, so I am not going to declare anything about the subject. But here is a bit of Aruch haShulchan on the issue.
See Aruch Hashulchan, siman 362, seif 7-11, and particularly seif 11. And see siman 374.