Authorize.net ARB stopped working
It's hard to answer definitively without knowing your version of CiviCRM.  However:


If you're using CiviCRM 4.5+, IPN transactions (the silent post URL) are stored in civicrm_system_log.  You can also replay them.
You should search your web server logs (Apache/Nginx/etc.) for references to the IPN URL.  If you're not seeing them, then Authorize isn't talking to your server.  If they ARE there, check to see whether there was a 200 (success) response or another response (e.g. 404 error).
Try visiting your IPN URL in your web browser.  Do you get a response along the lines of "The transaction has failed", or do you get something like your site's web page?  If the latter, you may have a broken redirect rule in your web server settings.