Do you know what version you updated from? 5.4.1 was a recent emergency update to correct an issue that the 5.4 update caused on some iOS devices, and that issue was indeed the loss of all images and presets in Lightroom Mobile. Is is possible that the phone was updated to 5.4 before being updated to 5.4.1? Only 5.4 could cause that problem, 5.4.1 was created to prevent it happening to those users who had not yet updated to 5.4.
All that aside, I'm the bearer of bad news.....if you did not have any form of backup of your phone (iTunes or iCloud for example), it's likely that all that Lightroom data is not recoverable. See this Adone note: Lightroom for mobile (5.4.0) on iOS and iPadOS