When one of your accounts exceeds the bandwidth (transfer) limit:

1. The site and all files on it will become inaccessable through a browser.
2. New mail sent to the account will still get through provided the account has enough space for it.
3. FTP will still be accessable.

To get the site working again:

1). Log into WHM. Click the "Limit Bandwidth Usage" link. Select the domain or account username for the account that is over its bandwidth limit. Click the "Limit" button. Raise their bandwidth limit and click "Change".

(2). You will need to unsuspend the account, as the system will have suspended it for going over the data transfer limit. You can unsuspend the domain by clicking on the "Suspend or Unsuspend Accounts" link. It's all pretty straight forward from there.

There is a delay from when you unsuspend it and to when it becomes active again. Be sure to check your domains daily, to make sure they don't go over their limits and you end up having to wait like this.