I would first check that wireless is switched on -- most laptops have a physical switch on the side or front of the case usually with an indicator LED.
Then go to Windows Control Panel, System, Hardware, Device Manager and see if the wireless network adapter is present or whether it is Yellow Flagged as faulty. You may need to delete the device and re-install it -- perhaps take the opportunity to download a more recent driver set from the laptop or adapter manufacturer's website.
Obviously also check Windows Network Connections etc.
I am having this same issue. This morning everything was fine; suddenly no networks are available.
I see my wireless LED indicator and it is off. I don't see a button for it anywhere. We have an HP Compaq 6710b. I looked followed your directions all the way to the device manager, fihart. Once I got there though I wasn't sure what I was looking for. Would the wireless network adapter be in a subfolder or all on its own. What exactly might it be called?
First I'd check the HP/Compaq manual because the wireless control switch may be a keyboard combination if it's not a physical switch.
In Device Manager, wireless adapter should be under Network Adapters -- there will probably be an ethernet port (the socket for wired networking) -- plus a wireless networking device (if it's switched on).