Try pinging FB by opening command prompt... type Ping www.facebook.com and see if your computer is talking to FB. If it is type the IP address into your browser. If you can get to FB that way then you probably have some sort of DNS issue. See if you are having problems with other web sites that you visit.
Out of curiosity, are you at work? At a corporate location, perhaps? It sounds like maybe Facebook is blocked.
you could try manually flushing your dns cache and getting a new IP address
from an elevated command prompt type: