Trademark I have owned and used for 9 years has been registered as a service mark in my state by someone else. Is this legal?

You own the mark, since you were the 1st to use it in commerce. But your rights are limited to the geographic area where you use the mark, because your rights are "common law" rights, and based on CA state law.

But since you didn't federally register your mark, someone else, like this competitor, can federally register your mark.

Your remedy is to seek to cancel their mark. You have to prove that your use was 1st, and certain other things. Petitions to Cancel can be tricky, so you're best off hiring a qualfied TM lawyer to help you.

In the meantime, you may also want to apply for your own mark's registration.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

Yes, your domain name will help to establish your priority of use. They may initiate a UDRP proceding to claim that your domain name is cybersquatting on their trademark, and then you'd use the same arguments in defense to that claim as you would in seeking to do the same thing to their domain name and to cancel their trademark.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

As Ms. Kosyln notes, you need to speak with a trademark attorney. There are just too many facts yet to be disclosed that will be important in order to determine who now owns the rights (if any) to the name of the event.

One fact that will be important is whether you "abandoned" the name -- your question does not make clear if you've held the event every year for the last nine or just once nine years ago or any of the years in between. If only nine years ago then the name (if protectable) may be free for anyone to use. We also do not know the name of the event (do not tell us). That's important because the names of many events are not protectable under trademark law ("The Sacramento Fair" would not be protectable, for example). Note: even if not protectable under trademark law, you may have an "unfair competition" claim.

Your analysis boils down to an economic question: will you be better off (financially) spending your money on a trademark attorney (with litigation experience) to try to stop the other event from using the same name as the one you have used or would you be better off spending your money re-branding your event (which you may just have to do anyway)? Good luck.