Is the name of the network (SSID) hidden? If the router was setup to not broadcast the SSID, the network will not be visible. Try checking the router settings or creating a new connection and adding the SSID and password manually.
Another possibility is that the wireless adapter was set to 802.11a for some reason and therefore, it would only work at 5 GHz. Make sure the wireless mode is 802.11a/b/g.
Based on the description I believe those are the only two possible causes but let me know if you need further assistance.