Some more information would be useful, have you checked in device manager to see if there are any missing driver or conflicts. Is wifi button switched on.
Additional to what urie said, could you tell us the model computer, the name of the WiFi adapter and also the Driver version that's currently installed
The possibilities are: A driver issue. A network filter issue (such as used by Antivirus products) or firewall issue or soft switch issue (such as an application used to disable or control radio status). A hardware/firmware switch may have been set to disable the local Wi-Fi capable device(s) Hardware failure Access instead in being provided thought set set to be provided though a wired connection No hardware capable of connecting to Wi-Fi Unavailability of remote Wi-Fi device