This is one of the more frustrating problems I have encountered with Magento. It generally occurs after working with 3rd party plugins. It will lock you out of the both the front-end and back-end of your website.
The solution, the only one as far as I know, is to login via FTP to the base of your magento installation. Look for a file named maintenance.flag and delete this file.
This should most likely solve your issue and clear the 503 error.