Eruvin Made Easy part 10: When the Eruv Goes Down on Shabbos

If the eruv is definitely down and was definitely up when Shabbos begins can you continue to carry? Do we say כיוון שהותרה הותרה for a city eruv like we do for an eruv chatzerot? The opinion of האלף לך שלמה

If you discover that the Eruv is down should you tell others?