When does Gmail block your account?

One of the possibility why you can’t log in to your Gmail account is because of it having been locked by Google. This happens automatically when “abnormal usage” indicates that an account has been compromised. According to Google, it may take up to 24 hours before account access is reinstated.  Reasons for locked accounts include:

• Receiving, deleting, or downloading large amounts of mail in a short period of time. If you're getting the error message, 'Lockdown in Sector 4,' you should be able to access Gmail again after waiting 24 hours.
• Sending a large number of undeliverable messages (messages that bounce back).
• Using file-sharing or file-storage software, browser3 extensions, or third party software that automatically logs in to your account.
• Running multiple instances of Gmail.
• Browser-related issues. Please note that if you find your browser continually reloading while attempting to access your Inbox, it's probably a browser issue, and it may be necessary to clear your browser's cache4 and cookies.

Another possibilities of why you can't log in to your account are:

1. Login problems.
2. Forgotten username or password.
3. Hacked accounts.