All my iMessages were deleted
Reboot your phone.
I discovered a solution for this one accidentally, after I encountered this bug a couple of days ago. I turned my phone off for a time, and then back on again - when it was turned back on, the list of messages was fully restored. I think that this might be because only the listing of messages was made inaccessible, and the messages themselves weren't deleted - I noticed that when I was texted by someone who I'd received messages from in the past, the message history was properly displayed once I opened the text conversation with them.
It's possible that the simplicity of fixing this issue is why Apple hasn't patched the bug in the 5+ years it's been known about.