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I am sorry to learn of this experience. The easiest way to file a complaint is to do so with the US Dept. of Justice here: http://www.ada.gov/filing_complaint.htm
The CA ADA does allow for a private attorney general's right of action. This means you can file suit for violation of the ADA. If you do choose to do this there are attorneys in CA that make a specialty of suing businesses for ADA violation. I can't tell you what kind of damages you would hope to claim out of this (it really would depend on the facts of the matter - the focus is on the violation (distress, embarrassment, etc. are not legally compensable damages, and are the issue). but if you find that the elevator had been out of service frequently, or for a long period of time, there may be some money in it.
(Most of the money in these suits does go to pay for attorney's fees, the public purpose of these suits is to encourage businesses to keep up to code as opposed to creating high profitability suits, but you can certainly contact one of these specialists and discuss your case in more detail).