Look at the Device Manager > Universal Serial Bus controllers and see if your device shows up at all. It will either show up under its brand name or "USB Mass Storage Device". If it doesn't show up, it may be dead. If it does show up, there is a troubleshooter that will help you diagnose any problems.
Also, take a look at the USBVIEW program from the Windows Platform SDK (http://www.microsoft.com/msdownload/platformsdk/sdkupdate/default.htm). USBVIEW will show you everything that Windows knows about a USB device, if it is able to communicate with it.
Two sites with good info on USB troubleshooting are:http://www.butterflymedia.com/TechSupport.asp