This morning when i tried logging to my gmail account through web browser, it was saying ” The username or password you entered is incorrect 
” (some times this happens with Google Chrome).So tried in couple of other browsers and it was the same. Next i tried it in gtalk and a new message was shown “Account Locked“.
Google explanation is
We detect abnormal usage that may indicate that your address 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.
Unusual activity includes, but is not limited to:
- Receiving, deleting, or downloading large amounts of mail via POP or IMAP in a short period of time.
- Sending a large number of undeliverable messages (messages that bounce back).
- Using file-sharing or file-storage software, browser extensions, or third party software that automatically signs in to Gmail.
- Leaving multiple instances of Gmail open.
- 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 cache and cookies more here
I then again tried logging to gtalk application and a new message appeard saying “This account whas been locked and requires unlocking” and a captcha appeard to unlock. After passing the captcha , the account was unlocked . What a crap of no reason my account was locked 