From your 'localhost' machine, go to Admin->Access. There you'll see the list of blacklisted or whitelisted addresses. If you enter incorrect credentials too many times from an address, it's possible you IP was blacklisted for a short period of time.
You can remove the blacklisting and/or add addresses to the whitelist for permanant access. The wiki has more information: http://wiki.snomone.com/index.php?title=Access
Also, make sure your Firewall has port 80 open, and the PBXCTRL is an exemption to the firewall (assuming you're on Windows)