Did you get a new modem/router? maybe that is blocking it

Not quite sure actually, I don't even know if modems can block things.. but look on the bottom of your router for the access address (it's a web site) or try using your IP (enter it into your browser the same way you would a web site). your router should also have your default login name and password on it, once you're in look around and see if you can if it's blocking ports and such... sorry I can't help more

also you could try going to your router/isp's site and search the FAQ

I'm pretty sure it's the same with every modem, but basically go to the url specified for your modem by your ISP (for example I have a D-Link modem so I go to http://10.1.1.1/ but I dunno if that will work for other modems (I'm not too cluey on it)).

Log in using the admin name and password given to you with the modem and then look for your UPnP settings and your Port Forwarding settings.
See if UPnP is checked, I'm fairly sure it should be but I'm not sure, if it isn't then check it and click whatever option restarts your modem.

If it still doesn't work then fiddle with port forwarding, make a custom rule allowing port 6667 (or port 6660 to 6669 to be safe).

Following that check your modem's firewall settings and basically enable anything that looks like it shouldn't be enabled.

Personally I don't really know what I'm talking about, I just tinker with things until they work.