OK first, Josh O's answer is well to be pointed about it, not good at best.
Since you didn't mention what kind of computer it was I am going to assume it is a laptop because you stated WiFi won't connect and 99% of the time when someone says WiFi it means a laptop to most people. Now this is going to sound really stupid but have you checked to make the wireless is not "physically" turned off on your system??
Let me explain. Most laptops today have some sort of of either hard or soft switch to turn the WiFi on and off, regardless of the state of the software settings. On my laptop for example there is no 'hardware' switch but I do have a key combination, soft switch, on my laptop keyboard that I can turn not only the WiFi off but also the blue tooth.
It usually involves a key toward the bottom left of the laptops keyboard labeled Fn
Make sure you are not using an external keyboard because it will NOT have this key. Now look at the function keys across the top of the keyboard, the ones that say F1, F2, etc. There is one that should have some type of WiFi signal icon printed on it. I can't tell you exactly what it will look like because there are a lot of different ones. Mine shows a radio tower with radio waves coming off of it. It could also just look like a WiFi signal.
Basically what you want to do is first figure out if you have basic function keys turned on or the "special" keys turned on. You know, sound up, sound down, mute, etc. The easiest way to check is if you hit F1 when you are on the windows desktop and the Windows Help screen opens up, then you have function keys turned on. If this is the case then hold down the Fn key, the one mentioned above, and press the WiFi signal key once. Don't hold it down though. This should enable the WiFi if it is turned off. If you have the special keys turned on, F1 didn't bring up the help screen, then you can press the WiFi signal key by itself to try and turn it on.
If you have a hardware switch then it will most likely be on the from on the laptop under the front lip edge.this placement almost always causes someone a problem because they hit the switch without even realizing it and turn their WiFi off and don't understand what happened.
One thing i should have mentioned at the beginning, open device manager and see if your wireless card is listed under the networking section. If it is not then it is most likely physically turned off and the above procedure should turn it back on. If it is listed and the above procedure doesn't work then you might need to update the driver. Sometimes the driver can get hosed up and will cause all kinds of screwed up things.
The last option is that either the internal WiFi antenna some how broke of the WiFi card in the laptop itself broke for some reason, either way these will cause you to have to have the laptop serviced.
Hope that helps.