UPDATE: Cloud 9 decided to revert back to their previous url structure as it caused worldwide issues.
At some point over the weekend Cloud 9 updated their preview url from https://sitename-accountname.c9.io/ to https://sitename-accountname.c9users.io/ which caused all of the WordPress installations to break. Here is the Cloud 9 error fix to get your WordPress sites back up and back to developing.
To start you are going to need to change the name of your WordPress installation by adding the following code into your functions file:
Code provided by the WordPress codex.
Now save the functions.php file and you should be able to log into your WordPress dashboard. Next go to the Settings tab from the left menu on the dashboard and click on the General tab. From there you will see that the WordPress Address (URL) and the Site Address (URL) are grayed out. Go back to your functions.php file and take out the code we added in the previous step. Go back to the dashboard and refresh the page. The WordPress and Site Address should un-gray and now you can enter the correct url for your site preview: https://sitename-accountname.c9users.io
If you refresh and your dashboard breaks don’t worry! You can still scroll down and find the WordPress and Site Address fields, update and save them. Make sure that users is added behind c9.
Let me know if you have any issues fixing your site.