If the button or on/off switch that is provided for the WiFi (may have a line with three curved lines coming off of it as a symbol for the button) is toggled or enabled and the wifi won't turn on, then there is a good chance that the driver for the wifi isn't loaded.
You can tell if the driver is missing by opening the Control Panel, opening System, opening Device Manager and looking for a Network Adapter category. Expand the Network Adapter category by clicking the plus sign and look for a WiFi adapter in there. If it has a yellow exclamation point ! on it then there is a problem with the network card and it needs to be reinstalled. If there isn't a WiFi or Wireless Network adapter in there, chances are it is found under a category called "Other Devices" and it still needs to be reinstalled.
To install it, you will probably have to delete the device out of the device manager (highlight it and press the delete key on the keyboard) and then plug your laptop into wired network connection (usually your router or internet service modem will have a network port that you can plug into directly with an ethernet cable). You'll go to the manufacturer's website, click on support and search for drivers for your laptop. Be sure to go to the network section and download and install the network driver for your system.
For your particular model, you can start here: HP Pavilion zd8000 CTO Notebook PC Drivers