If your conversation history has disappeared, the most likely explanation is that you're not currently signed in - or that you've signed in with a different email address from the one your account is registered to.
Check you're signed in
Conversation history is tied to your account. If you're not signed in, you won't see it. Look for your name or an account indicator in the app.
If you used Solomon anonymously before creating an account
Anonymous sessions are not saved. If you had conversations before creating an account, those conversations are not recoverable. Only conversations after account creation are stored.