My computer running Ubuntu 12.04 can't connect to a specific WiFi SSID, but it can connect to any other WiFi SSID except the specific WiFi SSID.
In addition, my Android phone can connect to my specific WIFI SSID,
I borrowed a computer with Windows 7 installed to do a test, it can also connect to my specific WiFi SSID.
I don't know why. It shouldn't happen like this and not a long time ago, I was able to connect to the specific WiFi SSID in Ubuntu 12.04. I don't know what I did to cause this problem.
More details: In Ubuntu 12.04, I can't find my WiFi SSID in the list of common WiFi SSID list, it should be there. But I can find my WiFi SSID in the hidden SSID list, I am sure that I didn't make the SSID hidden. More strangely, I tried to connect the hidden SSID in ubuntu and it can't connect to the specific WiFi SSID. One of the evidence is that I can find the specific WiFi SSID both in my Android phone and Windows system.