Why has my Gmail account been locked?
If we detect abnormal usage that may indicate that your account has been compromised, we may temporarily disable access. It will take between one minute and 24 hours for access to be reinstated, depending on the behaviour detected by our system.

Abnormal usage includes, but is not limited to:

Receiving, deleting, or POPing out large amounts of mail in a short period of time.
Sending mail to a large number of recipients in a short period of time.
Using 3rd party file-sharing or storing software in your account.
Logging in from multiple locations in a short period of time.
Using your account for purposes other than email.
If you're using POP to retrieve your messages from Gmail and your account has been locked, please read I enabled POP access, and now I'm locked out of my Gmail account.