The Consumerist reports that Google has accidentally disabled a large amount of users accounts in an attempt to target a large network of spammers to keep them out of the Gmail system and keep your inbox free from spam. Now they’re actively reinstating these accounts ASAP.
There are actually a couple of reasons, like logging in from multiple locations quickly, or sending mail to many people in a short period of time - basically anything that makes you look like a spammer.
If your account gets locked out like this, you can file a report here. Reinstatement are said to usually occur with 24 hours, but that's small solace for people running their job, lives, or brain from their Gmail.
Source:→ ConsumeristGoogle, Gmail, Whew, Accounts, Disable, Disabled, Lock, Locked, Activate, Activation, Oops!