Have you tried to disconnect from the wireless connection in the XBOX 360's settings and just tried to re-set it up? If you haven't please do by going to My XBOX > System Settings > Network Settings > Highlight your Wireless SSID and press Y to Disconnect.
The XBOX 360 should re-scan for Wireless SSID's in the area and it should find yours again, press A when it does and see if it asks you for your wireless key. If it doesn't, try to go into your routers wireless control panel and change from WEP if possible
to WPA2 (I say WPA 2 because that is what I currently use and it's been working perfectly fine)
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