What version of Windows is installed on that PC?
Please define "won't go online" and tell us if you are using wifi or a wired connection.
You problem could be driver related, malware infection or hardware.
The first thing I would look for it to check if a "proxy server" was installed by malware.
The next thing I would do is to delete the network hardware and then restart the computer. Windows should then reinstall the hardware and reestablish all the software connections for Internet access. Often this will fix your problem.