The key you put in has to match the key from your router, and it has to support the type of encryption used by the router. For example one of my routers uses WPA-PSK and another uses WPA2-PSK. Since my laptop supports both, I can connect to either when I'm in range. Did your computer EVER work with this WiFi, or are you talking about a totally new setup?
It's also case-sensitive, which means you must use capital letters or small letters as they appear in the correct password which you'll find in router's configuration utility or printed on the router casing (unless you've changed the default).