Text messages disappearing on iPhone and iPad

Why are my text messages disappearing? When I go to re-read or send a text to some people, there are no previous texts under their name. Just a blank screen. Also a few texts now have a date of 2000, but I didn’t even have a phone then. What is going on and how can I fix it?

PS this is happening on my phone and iPad but this web page wont let me select my phone.

[Re-Titled by Moderator]

Original Title: Text messages have vanished.