all my photos are GONE

I doubt if you would have much luck, but you can try this if you have an iOS device:

https://helpx.adobe.com/uk/lightroom-classic/kb/recover-lightroom-mobile-photos-itunes-file-sharing....

Alternatively, do you back up to iCloud, and if so one option could be to wipe your device and restore from a backup previous to the Lightroom update?

This though may not be as smooth as it sounds and could lead to other non-Lightroom issues.

Or maybe use a spare device and restore from iCloud and see if that works?

Unfortunately I cannot bring good news.

Here are the links to Adobe's documentation and the thread of the forum that is monitored by the engineering team:

https://helpx.adobe.com/lightroom-cc/kb/lightroom_mobile_5_4.html

https://feedback.photoshop.com/photoshop_family/topics/lost-my-pictures-and-presets-after-update-today-18th-august-2020?topic-reply-list[settings][filter_by]=all&topic-reply-list[settings][page]=1#topic-reply-list