Unexpected logouts are usually nothing to worry about - just a session that has expired. Your account, memory, conversations, and Journal are all safe.
Why this happens
Sessions expire periodically for security reasons. It can also happen if you've cleared your browser cache or cookies, or opened Solomon in a private or incognito window, which doesn't persist sessions.
How to get back in
Go to the Solomon sign-in screen, enter your email address, and wait for the magic link. Click it and you're back in. Sol will greet you by name and everything will be exactly as you left it.