To start off, take a look at your router. There should be a "Wifi" led on the front panel indicating the ststus of the wireless function on your router. Typically it should be a green led indicating that it is enabled and transmitting. If it is disabled or malfunctioning it may be yellow, orange or red.
If this is the case a power cycle may solve it without any further action.
Otherwise you will need to enter the router configuration/setup and enable or configure wireless. Since you can see other available networks in the range of your pc, there is most likely a problem with your router and not your pc.
If on the other hand wireless is enabled and working, it may be your wireless adapter but I find it unlikely.
Thank you very much for the help. Yes, the wireless was disabled and I enabled it and it is working now. The leading question is: It was working alright but all of a sudden it didn't. I did not do anything to the router settings; so, how did it disabled itself?