cannot receive new emails Mac OS X 10.8.2-comcast.net
Cannot Receive new emails since 1/1/13 Comcast.net email acct
MacBook Pro (15-inch 2.53 GHz), OS X Mountain Lion (10.8.2)
It would likely be that the POP or IMAP paths are misconfigured, or possibly Comcast has changed settings or requirements, or there's a corruption of some sort with either the Comcast set-up or with your account set-up within Mail.app.
As a first step, invoke the Connection Doctor tool (Window > Connection Doctor) and see what that reports.