According to a Google Groups thread, Google accidentally deactivated a batch of email accounts. Gmail Guide of Google writes in to inform the community of the issue:
We have been actively investigating a batch of accounts that were accidentally disabled and are currently in the process of re-enabling these accounts. This error occurred in an effort to target a large network of spammers to keep them out of the Gmail system and keep your inbox free from spam. We apologize for this inconvenience and appreciate your patience as we re-enable these accounts as quickly as possible.
As numerous users reported problems with their accounts, Gmail Guide posted an addendum:
We have been diligently working for the last few hours to identify affected accounts and we have now re-enabled access for most of these accounts. If you are still seeing a message indicating your account has been disabled, please be patient and you should be able to login again very soon.According to the affected users, Gmail gave errors such as "Sector 6" or "Account Lockdown." Many users had their accounts reactivated within the first 48 hours, but others had to wait about 6 days from one of the most recent messages on the thread.
We apologize for any inconvenience this may have caused.
The issue has not been addressed since Friday, but if you are encountering any similar issues, you may contribute to the discussion at Google Groups.