Gmail is a popular web based email service from Google. It can be really frustrating when a user is not able to access his Gmail email account. At times, Gmail account could be locked by Google due to various reasons. Once Gmail account is locked, it is automatically re-activated or reinstated within 1min to 24hrs. So if your Gmail account has been locked wait for upto 24hrs or else contact Gmail support.
Why Gmail account may be locked
1. Receiving, deleting, downloading large amounts of mail via POP or IMAP.
2. Sending a large number of undeliverable messages (that bounce back).
3. Using plugins or software that automatically logs in to your account.
4. Browser issue, try using different browser to login.
5. Opening and leaving multiple instances of Gmail open.
Either of above could be reason of Gmail account locking up. Majority of times, it is heavy Gmail account activity (reason 1) that results is Gmail account locking! [via Gmail help]