On your computer's side:
Navigate to \Windows\system32\drivers\etc\ and open the hosts file. (extention less text file)
If you have a line with 127.0.0.1 www.facebook.com, pus a "#" in front of it or remove it.
If you have a firewall installed, check it's configuration to make sure it allows access to facebook.
If you access the internet from a local network, then, the network administrator may not want the users to access facebook, and probably other social networks. It can also include several gaming sites, like WoW and others.
Probable from a school or workplace network. In this case, you must comply with the desires and requirements of the network owner.
At school, you may politely ask the network administrator if you can access facebook. If he agrees, it will probably have time/days restrictions, like only outside of class hours and weekends. It may even include locations restrictions.
Electro, September 2011