You can try this one if you can't sign in [1]. You can also check out their help page or social media as mentioned above. If you make enough noise they'll likely respond to you faster, just like most businesses. I believe Coinbase was trending just a few days ago because a user reported they suddenly got locked out without any reason at all. I'm not following that case closely but I'm sure it got retweeted thousands of times and you can do the same if you want to. Just mention a popular user if your tweet fails to get any reaction at all.

[1] https://help.coinbase.com/en/unable-to-sign-in?product=lost-access