I've heard others say the data is there. Connect your phone to a computer or SD card into an adapter and back up your files to the computer. Format the SD and transfer the files back. A pain, but I haven't heard of anyone completely losing their info.
Please try the suggestions made by @MrClean69 . If the issue persists, please send us an error report. We apologize for the inconvenience and appreciate your contribution to the beta program.
One UI Beta Team.
I've been on 3.0 Beta a week now and my SD card corrupted out of nowhere 2 days ago. A fix I did was put the card in another phone and the phone read it then put the card back and was good to go again