What router do you use?
What encryption are you using?
Does the wireless work with other devices?
Are you sure the wifi network you are connecting to is actually yours and not someone else locally with a Plusnet router - try changing the SSID so you know it's yours.
While you are in the router check the wifi password - remember this is not the same password as your plunet account, it is a password set in the router for the wifi connection. If you are not sure what it is, put in your own password. You can also check what the default encryption method is (WEP, WPA/PSK, WPA2/PSK etc).
