Follow these steps if your wifi no longer works.
disable firewalls that may block software and/or the wireless interface
Find the keyboard shortcut. Occasionally this is FN + F#. The F key usually has a picture of a antenna, or a series of semi circles inside one another
click start - settings - network connections and look for your wireless network connection. Right click and select “enable” (if not enabled)
click start - settings - control panel - administrative tools - computer management - system tools - device manager. Look under network adapters - is there a wireless card there? right click - select enable
Restart the computer and enter the bios - look for a section called devices and make sure “wireless lan” is enabled. (Some bios' don't have a devices section)
boot a linux live CD and find out if wireless works
upgrade wifi software to latest release
download drivers from the manufacturers website, and reinstall
If you had just upgraded (before the problem appeared), go onto manufacturers site and revert back to old drivers. You may need to uninstall the other drivers and restart first.
reinstall windows
purchase an external usb wifi card.
(remember if one of these steps has been implemented, you should follow the list from the start again)