There may be several reasons for your Google Account to get locked. Some of them include- downloading large amounts of mail via POP or IMAP, using a file sharing software that automatically logs into your account, sending a large number of undeliverable messages, etc.
Well, if your Google Account is locked then you may try to unlock it by going to Google Unlock captcha page. In this page you need to enter your login details, the captcha as shown in the box and click on the ‘Unlock’ button. I cannot assure you that it will get unlocked. But, there’s no harm is trying.