Going with the whole Occam's razor approach, have you tried calling your ISP yet? It could be an issue they are experiencing that is affecting your connection. If they test your line and everything seems okay, best solution would be to try and get another computer to test if that can connect (borrow a friend's laptop or something).
If you do manage to establish the fault is with your computer system, you've already tried another adapter without success (hopefully using a different cable too). Are you connecting through a router or a server (or another way)? Need to check you can still ping this device. Also, try ipconfig /all, tell us what you see (apart from the external IP number).
