If your Teams chats, activity etc disappear without warning, do not panic.
Firstly, try signing out of Teams and signing back in.
Next, quit the Teams app and relaunch it. Do the messages/conversations re-appear?
If neither of the above resolve the issue, the next step is to uninstall and reinstall Teams - https://freshservice.domino-printing.com/a/solutions/articles/50000071872