It could be problems with your ISP, although if that were the case I would imagine other programs would be experiencing problems too (including Internet Explorer) Start > Control Panel > Network Connections
Click on the Internet connection you're currently using and see if the IP settings are correct. Right click it, and click 'Properties' and see if everything seems OK there. If you're unsure, phone up your ISP and check with them. Whilst you're talking to them you might want to inform them of your minor plight and see if they can help you. It could be that some sort of hardware settings were uninstalled or reset, especially if you're a cable user.
You should by now have read and followed the instructions posted on my previous link to Merlin's post about common IRC error messages. To make sure, do them again "Check the obvious first: check that the destination address is a valid IP address. If you used a hostname, did it resolve to the correct address? If the hostname resolution uses a local host table, it's possible you resolved to an obsolete address. Can you ping that hostname?
Do you have a router configured? Is the router up and running (check by pinging it, and then ping an address on the other side of it)? Try a traceroute to the destination address to check that all the routers are functioning.
Check your subnet mask. If you don't have the proper subnet mask, your network system may treat a local address as a remote address (so it forwards addresses on the local subnet to the router, rather than broadcasting an ARP request locally), or visa versa.
Your Subnet mask should be visible when you click on your Internet connection in the 'Network Connections' area I mentioned earlier in this post.
I hope you manage to get your problem sorted soon :-)