Well, I personally have a PS2 so I'm not very familiar with the setup of the xbox adapter. Basically, try shutting off the xbox and leaving it off. Then unplug the modem (by the way...is it a cable or DSL modem?) Leave the modem off for 30 seconds and then plug it back in. Then after it's on for about 20 seconds or so turn on the xbox. See if you can't get online then.
Also, you may want to throw the xbox network adapter's CD into the drive and make sure that your adapter is set for "Auto Configuration"