Hi,
make sure that your Wifi is switched on and the symbol is displaying on your PC, some people think that just switching on the wifi on the router is enough, I assume you are on a notebook!!.
Make sure that Wireless connections are enabled in the network manager, you can check this on the network icon on the taskbar.
It should be left on "Connect Automatically and for "All Users!!.
1. type "iwconfig" in a terminal, it must list down your Wireless interface if it is working.
iwconfig shows
wlan0 IEEE 802.11abg ESSID:""
Mode:Managed Frequency:2.412 GHz Access Point: Not-Associated
Tx-Power=15 dBm
Retry min limit:7 RTS thr:off Fragment thr=2352 B
Power Management:off
Link Quality:0 Signal level:0 Noise level:0
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0
(the above is an example)
> 2. You can also do lsmod and check whether it is loaded or not
> 3. Check your lspci output for your network controller.
Regards Wolfman