Please note, I am not a CA attorney, so this post should serve as general guidance only. Some firms have a practice of safeguarding the original will for the clients, so calling the attorney who prepared the document would be the first step. If you cannot locate the original, you may be able to probate a copy of the will, but this usually requires court approval, and since I am not licensed in CA, I cannot guarantee that process is available (though it is in VA).
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DISCLAIMER: THE INFORMATION PRESENTED HERE IS GENERAL IN NATURE AND IS NOT INTENDED, NOR SHOULD IT BE CONSTRUED, AS LEGAL ADVICE. THIS POSTING DOES NOT CREATE ANY ATTORNEY-CLIENT RELATIONSHIP BETWEEN US. FOR SPECIFIC ADVICE ABOUT YOUR PARTICULAR SITUATION, CONSULT A QUALIFIED ATTORNEY.
A copy of a Will can be admitted to Probate if the original cannot be found if this involves the estate of somebody who has already died.
If the Will is for somebody who is still alive and legally competent to sign a new Will, then the lawyer who prepared the Will should be able to create a new original that can then be signed again, replacing the one that cannot be found.
Something to consider is whether or not a Will is sufficient for the person in question. For example, if the person owns real estate or has assets in excess of $100,000, a living trust might be a better alternative to a Will.
The above answers are correct. If the person has died and you are seeking to admit the will to probate, then the copy can be used if the original is lost. If the person has not died, and you are looking for the original, then you should check with the person's attorney or CPA who may have copies, and check in the places where the person kept their important papers (safe deposit box, home safe, home file of papers, etc.) If the person who made the will is still alive and has not lost capacity, then he or she can sign a new will to supersede the old one. You should consult with an experienced probate attorney, especially if your question arises in the context of seeking to administer the estate.
Disclaimer: This general response is not intended to create an attorney-client relationship.