The U.S. has been in compliance with the Berne Convention for many years, pursuant to which ownership of copyright does not turn upon formalities like publication with notice. As soon as someone creates an original work of authorship and fixes it in a tangible medium of expression, it is protected by the law of copyright, and the remedies afforded by the law of copyright to unpublished works. Thinking out loud about it, the common law of copyright may afford some protection too.
If the work is lost or stolen and you haven't a copy, there may be some practical obstacles to utilization of federal statutory remedies, including the conundrum of proving that it's your work when you don't have a copy, and registering the work with the Register of Copyrights at the Library of Congress (so that you can bring suit in federal court) when you don't have a copy to register. The link below may be helpful.
At a minimum I would suggest--if you're really worried about this--making a copy every time you incrementally update your diary. I would think making a copy each time would obviate the conundrums I described above in the event the work is lost.
Not legal advice as I don't practice law in California. It's just my two cents on the facts you describe in light of general principles of law. If you need legal advice, please consult a lawyer who holds California licensure. That's not me.
You most likely have a copyright in your personal diary. You obtain a copyright when you create an original work and fix that work in some form. An original work is one that is independently created by you and displays a minimum level of creativity. If your diary is independently created and displays a minimum level of creativity, you obtained a copyright as soon as you wrote the words down on paper. This would prevent others from publishing it.
However, a prerequisite to filing a copyright infringement claim is registration of the copyright with the U.S. Copyright Office. This would require you to deposit a copy of your diary with the U.S. Copyright Office, however, and you may not be comfortable with that. If someone actually stole your diary, you may also have some type of tort claim(s) against them, such as trespass or invasion of privacy. If this is the case, you should speak to an attorney about your rights.
No attorney-client relationship implied or accepted without a signed fee agreement. This response is theoretical only and for purposes of discussion. Attorney is not liable for any opinion expressed herein.
My colleagues have already fully responded to your question. I'll add only two points: Except for sound recordings fixed before February 15, 1972, there is no longer any such right called a "common law copyright" [17 U.S.C. 301] and, though you must deposit a copy of your diary with the Copyright Office to register its copyright, the only way for anyone to read your diary would be to "inspect" it at the Copyright Office -- and they would not be permitted to make a copy of it until you file a copyright infringement suit against that person [37 CFR 201.2(d)(2)].
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.