Ethernet not working on Mac OS X running Tiger

I'm sorry if someone has already posted something like this-I'm new to the forum so I haven't had time to look and just wanted to put this out there.

I'm using a macbook white, purchased new from Apple in 2007. I have the Tiger operating system, version 10.4.11. I go to college at Michigan State and live in the dorms and since I don't have wireless set up in my room, I use an ethernet connection.

It's been working fine all year and all day today, until tonight. I opened my internet browsers--I use Safari--and after about ten minutes, the browser said that the page could not be opened because I was not connected to the internet.

I proceeded to Network Diagnostics, and before I could do anything, it started scanning, and then said the "Connection to the internet appears to be working. Check the web address as it may be typed incorrectly."

Well, I'm running off bookmarks that I use every day, and I can plainly see that www.yahoo.com is not typed incorrectly.

System Preferences says that my ethernet is on and connected and running properly. Network Diagnostics recognizes it. But it will NOT connect.

I've restarted my computer, shut it down for a half hour, switched ethernet cords, unplugged from the box on the wall to the router and back again. My two roommates are both plugged into the router and work fine--mine, when plugged into the router, did not work.

PLEASE PLEASE help me. I've tried everything and cannot get this to work. I couldn't find a problem like this on the internet anywhere. Please help fix this!