[*]Can you please tell us the exact make and model of your machine.Also what operating system are you using? xp vista 7
[*]Can you connect in safe mode with networking.
[*]Have you tried each user on the machine,if you only have one user on the computer can you create a new and see if it will connect that way.
There are many reasons why a machine will sometimes not connect to the internet.Here are a few things that might cause such an issue.
1.Sometimes drivers just become corrupted and an easy way to fix this is to open your device manager and expand your network adapters and right click the drivers and select uninstall.
2.Sometimes firewalls and antivirus applications can block access to the internet you can surely find information about this on google,the easy way to remedy this is to uninstall your antivirus and firewall,(only try and uninstall your firewall if you are using a third party firewall you will not be able to uninstall windows firewall) and then reboot your machine and check your connection.
3.Often times DNS is an issue when dealing with a no internet situation. There are a lot of free dns servers available out there and changing the dns is pretty simple. Just remember to open an (elevated if using windows 7 or vista) command prompt
enter the commands ipconfig /flushdns
and ipconfig /register dns
after changing the dns on your machine.http://theos.in/wind...ns-server-list/
I hope that one of these methods will help you if not there is still more that can be done just let me know and I will assist you further.