active sync says that? or is that popping up in a little balloon down by your clock? have you tried different cables, or different ports or even a different computer? you may also need to uninstall active sync then reinstall it, unless your on Vista... either way you also may want to try uninstall and reinstalling the USB drivers.
some other good info in this thread
http://forum.ppcgeeks.com/showthread.php?t=22405
don't know if thats gonna help, you didn't exactly give us a whole lot of info to work with...