iMac connection trouble - help!

Hi there,

I bought my 24" iMac last week, got it set up and connected to my WiFi and all has been well.
Tonight, however, it spontaneously disconnected from the wireless router and refused to reconnect. My old laptop and iPod Touch both remain connected.
I have rebooted both the iMac and the router - this hasn't helped.
I have de-activated and re-activated airport.
In the bar at the top, when I click the airport symbol there is a tick next to my network as if I am connected, but when I try to open Safari I'm informed that I'm not connected.
I ran network diagnostics, and typed in the password for my network (double-checked). A pop-up appeared: "Your network configuration has changed. Click OK to proceed to the next step."
On clicking OK I got the following error message: Unable to join AirPort wireless network: "This computer was unable to join the AirPort Network you selected. You may ahve entered an incorrect password or network name, chosen the wrong type of wireless security, or you may be out of range of the base station. To change your previous choices click Go Back".
No error code given.

This occured spontaneously without me moving the router/comouter, changing any settings or passwords...

Any bright ideas? Any help much appreciated!
Ash

 

Irritatingly, connecting with ethernet is not an option right now... my router's by the front door, so would have to move the mac and lie on the doormat to use it, hehe. For now, I'm using my old windows laptop for connection.

I guess if i need to connect the mac to sort it out, then doormat it is.... but for now i'll put up with windows!