An issue has been identified in the latest version of Collector for iOS (10.3.4) that is available on Apple’s App Store. If you are currently using version 10.3.3 please do not upgrade to version 10.3.4. In the meantime, please ensure that automatic download of the app updates are turned off on your device. We are working on this issue and will have more information soon.
UPDATE: Monday 19th October 2015.
Collector for ArcGIS 10.3.5 update for iOS is back in iTunes App store: