Under California law, as the landlord, you have the duty to mitigate your damages. This means finding a replacement tenant. You are entitled to compensation from your former tenant for any rent that you were not able to collect from a replacement tenant.
Once you are able to quantify the amount of unpaid rent, you should next make a written demand letter to the former tenant asking for payment. An attorney, private investigator, or the post office can assist you in locating the former tenant's current address, if you do not have it.
If the former tenant refuses to pay, then your next step would be to bring a small claims court case against him or her. The maximum amount you can seek in small claims court is $7,500.
When deciding if you should sue someone there are two essential requirements. First, did the person who you think violated your rights have a duty to refrain from the activity that you think would form the basis of a suit or did they have a legal duty to do something and they did not do it. It is very difficult to determine the answer to that question based on the facts you list because it will depend on State law and possibly administrative law in your State and under Federal Law.
The second essential is where there compensable damages? Damages for which a Court can award you monetary awards or injunctive relief (order the other person to do something or stop doing something). If you have both of these elements you may sue.
However, lawsuits take a high degree of expertise and cost money. Many clients have come to me through the years and stated that the money did not matter to them, just the principal of the issue! When I tell them how much I and other lawyers charge by the hour it becomes obvious to them that the value of the lawsuit damages is very important.
You can seek out your lawyer referral service to seek counsel. There are agencies of the State and Federal Government you may want to contact. Talk to a lawyer before you decide to sue someone for expert legal advice!
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Disclaimer: This answer does not constitute legal advice. I am admitted in the States of New York, New Jersey and Massachusetts only and make no attempt to opine on matters of law that are not relevant to those three States. This advice is based on general principles of law that may or may not relate to your specific situation. Facts and laws change and these possible changes will affect the advice provided here. Consult an attorney in your locale before you act on any of this advice. You should not rely on this advice alone and nothing in these communications creates an attorney client relationship.