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A "non-refundable" deposit is non-refundable. (There are some limitations to this for very specific types of deposits, for example a "security deposit" placed on a residential (not commercial) rental agreement). But these exceptions do not extend to wedding or event venue deposits.
Furthermore, disputing a contract dispute that transpired 7 years ago is not going to be feasible as the statute of limitations is 6 years for disputing a breach of contract claim in the state of Washington.
You can try negotiating something with the venue to try to get them to cooperate with you (some sort of good will credit) but they are not obligated to provide you with any.