If you are using the iOS app Google Authenticator for our 2-step authentication process, there was a recent update of the app to 2.0.0 that inadvertently wiped all data upon upgrading.
If you haven't already updated your app, Google has pulled it for now and is reworking it.
If you have, you can add WordPress.com back by navigating to https://en.wordpress.com/settings/security/ and generating a new application password. In the Google Authenticator app you then need to enter wordpress.com (ignoring their @ prompt) and the code you just generated on WordPress.com. For more help, see http://en.support.wordpress.com/security/two-step-authentication/ .
If you cannot access your WordPress.com account in light of this issue, please go to https://en.wordpress.com/wp-login.php?action=lostpasswords and click on "Need more help?". On the contact form, be sure to note that you have updated your Google Authenticator app and need further assistance with your account.