These cases are difficult to defend and are attorney driven because of the ability to recover fees. Talk to a local lawyer to try and negotiate a resolution for you.

The problem with ADA lawsuits is that they are very easy to establish, and the costs of violations can be expensive. There are a lot of ADA abuses out there and what are called serial filers - people and attorneys that file multiple ADA lawsuits and settle for the violation amount - which is typically $4000.
Unfortunately, fighting an ADA suit is often more expensive than settling. Google ADA abuse and you will find some organizations that are dedicated to helping business owners fight these lawsuits.

You probably have been sued by an ADA mill. More facts are needed before a meaningful response can be given. However, chances are that you have been sued for violations of the ADA, and Civil code sections 51,52 (the Unruh Act) and Civil Code Section (54,55) the DPA. In 2012, Gov. Brown signed Senate Bill 1186 which put some restrictions before ADA suits can be filed. Whether or not those procedures were followed requires further information. Also, even if you settle, you will need to remediate and correct the alleged violations. With the new laws, you can get a stay of the lawsuit to give you time to remediate. Again, you will have to follow certain procedures to get the stay. Also, are you the landlord or simply renting space? If not the landlord, you may be able to get them to participate - but chances are that they too have been sued.