If you can't connect to the Master Server, here are a few basic things you can do to try and get connected again.

Also, are you having problems with any other multiplayer games or is it just LFS? Can you browse websites ok with no problems? This might help narrow the cause down a little.

Make sure that your firewall is allowing LFS access to the internet.
If you've updated LFS you'll need to allow access again (via firewall).
Check with your ISP that everything is ok. ISP network status pages, ISP settings things like, service throttling, traffic shaping or port blocking.
Check router/modem settings, you might to need allow programs access to the internet, just like with a firewall.

Few related Topics:
http://www.lfsforum.net/showthread.php?t=20433
http://www.lfsforum.net/showthread.php?t=17791
http://www.lfsforum.net/showthread.php?t=21445

If anyone has any other things or threads that people could try or look at can you please add them, try to keep it simple and easy to follow