Generally speaking, all connectivity problems fall into either of two categories:
Hardware or Authentication
The easiest way to determine where your problem lies is to find a free wi-fi spot (McD's or Starbucks, for example) and try to connect to their network.
The reason for this is because the networks are unsecured, meaning they don't require passwords and the encryption is typically WPA2 (default on all Wi-Fi routers/modems using 802.11 B standard or newer). An alternative would be to temporarily reset your router to its default settings (which changes the SSID, password, and encryption).
If you can't connect to any free Wi-Fi network, see if you can get your hands on a USB wireless adapter, and then try testing your...