One of my Raspberry Pis (a pi-zerow to be precise) suddenly was not able to see my main wifi router.
Restarting the Pi made no difference.
Looking around on the web. A suggestion was found to check the locale settings.
Selecting Preferences => Raspberry Pi Configuration Dialog. I noticed a "Set WLAN Country" Button on the Localisation Tab.
Clicking on the button displayed a select list. The list was set to US. So I changed this to GB (my locale). I then had to reboot the Pi and upon restart my router was appearing in the list again.