This problem might occur frequently with many users hence I am posting the solution. What happens is that the routers broadcast waves on some frequency and various channels. Some times the Wireless card which comes default embedded in our laptops is not able to scan the frequency of router or connect to the channel on which router is broadcasting. Usually this setting is done by router in auto mode. Unless some geek changes the channel on which router should broadcast which receiver at your laptop should be able to pick. Usually Auto mode on routers work perfectly. But in some cases like mine I found use of an external USB wireless adaptor like this one http://www.amazon.in/USB-wifi-Adaptar-dongle-wireless/dp/B00W97H75I?tag=googinhydr18418-21&tag=googinkenshoo-21&ascsubtag=46dcded8-0bea-4080-8701-f8ab33d4406c
solves this problem. In my case I have tried at various places I still have the laptops wifi which also scans the network but is not able to connect. But using external USB wifi it solves the problem.